How to Prevent an Unwanted 2027 ACA Auto-Reenrollment: The Dec 15 Deadline, Crosswalked Plans, and the Subsidy Recalculation Trap

Open Enrollment for the 2027 ACA plan year opens November 1, 2026 and the single most important date on the calendar is December 15. That is when the marketplace runs auto-reenrollment. If you have a 2026 marketplace plan and you have not completed an active plan selection by midnight on December 15, 2026, HealthCare.gov (or your state exchange) picks a 2027 plan for you, applies a recalculated advance premium tax credit, and starts billing you for coverage on January 1. In some cases that works out fine. In many cases — a plan that got discontinued, a benchmark Silver premium that shifted, an income estimate that stale — the January 1 invoice is a surprise, and the first appointment of the year reveals that the previously in-network doctor is suddenly out of network.

Auto-reenrollment exists to make sure nobody loses coverage simply because they forgot to log in. It is a safety net, not a strategy. In 2026, with the benchmark Silver crosswalk churn and the ongoing adjustments to the applicable-percentage table, the probability that the auto-selected plan is the right plan is lower than it has been in several years.

Here is the actual mechanism — the crosswalk hierarchy, the subsidy recalculation, the deadline — plus the five lines in the renewal notice that decide whether to actively re-select, and the paperwork for doing it.

On the morning of December 16, 2026 the marketplace examines every active 2026 enrollee who did not make a 2027 selection. For each enrollee it runs through a hierarchy. The hierarchy is public and documented, and knowing it is how you predict what you will be auto-enrolled into if you do nothing.

1. Same plan, same issuer. If your 2026 plan is being offered again for 2027 by the same issuer, you stay in it. New premium, new cost sharing, new formulary, same plan number. This is the most common outcome and the lowest-friction result — but note that "same plan" can still have a materially different premium and a 20 to 40 percent reshaped drug formulary.
2. Crosswalked plan from the same issuer. If your 2026 plan is being discontinued but the issuer still offers plans in your area, the issuer proposes a successor plan at the same metal tier with "comparable" benefits. The marketplace validates the crosswalk. This is where surprises happen: the crosswalk is a product decision, not an actuarial twin.
3. Lowest-cost plan in the same metal tier, same issuer. If the issuer leaves the market entirely in your rating area but still participates nearby, the marketplace maps you to its lowest-cost plan in the same metal tier.
4. Same metal tier, different issuer. If your issuer leaves your rating area completely, the marketplace selects the lowest-cost plan in the same metal tier from any participating issuer in your area. This is the most disruptive outcome: new network, new formulary, new member services number, new app.
5. Adjacent metal tier. If no plan is available at your current metal tier at all (rare, but possible in thin rural rating areas), the marketplace steps to the nearest adjacent tier and selects the lowest-cost plan there.

Through every rung of the hierarchy, the marketplace recalculates APTC using the most recent income and household data in your account. If you have not updated your estimated income since you first enrolled in 2026, that is the number the system will use for 2027.

Three things are shifting under the hood for the 2027 plan year that make a passive auto-reenrollment more likely to land in the wrong place.

1. Benchmark Silver reshuffles

Your APTC is anchored to the second-lowest-cost Silver plan in your rating area, the benchmark. When a new issuer enters your area with an aggressive Silver, or an incumbent re-prices, the benchmark can move to a different plan — and your subsidy follows the new benchmark, not the plan you are enrolled in. Enrollees who stay in a non-benchmark Silver can see premium changes in either direction without any change to their actual chosen plan.

2. Formulary rework around GLP-1s and the new negotiated-price drugs

Issuers rebuild their 2027 formularies around the second group of Medicare-negotiated drugs taking effect January 1, 2027, and around aggressive utilization management on GLP-1 agonists. Commercial ACA formularies are not bound by Medicare's negotiated list, but issuers often rework tiering across the entire book of business at once. A plan that covered your Mounjaro, Zepbound, or Ozempic on Tier 2 in 2026 may move it to Tier 4 or exclude it entirely in 2027.

3. 2027 FPL used in the subsidy calculation

Plan-year 2027 subsidies are calculated using the 2026 federal poverty guidelines: $15,650 for a single-person household plus $5,580 for each additional person. If your household size changed in 2026 (new baby, child moved out, divorce, a dependent dropped) and you did not update your account, the marketplace uses the old number and the APTC is wrong.

The marketplace mails a Marketplace Open Enrollment Notice and the issuer mails a plan-specific renewal notice. Together they contain almost everything you need. Read for these five lines.

If any one of those five lines flags re-select, log in to HealthCare.gov (or your state exchange) and complete an active plan selection by December 15, 2026. The portal walks you through updating income and household, then displays all plans available in your area with the recalculated APTC already applied. You do not have to leave the issuer you have today; you can choose the same plan actively, which produces a cleaner audit trail than letting the system assume for you.

From the moment Open Enrollment opens on November 1, you can actively select a 2027 plan. The entire workflow runs about 30 minutes if you have your paperwork in front of you.

1. Log in to HealthCare.gov (or your state exchange). Confirm the account is tied to the right 2027 application year. If the portal still shows your 2026 application as current, start the renewal application; do not create a new one.
2. Update household and income. List everyone who will be on your 2027 tax return and your realistic 2027 modified adjusted gross income. Err slightly high on income if you are uncertain — it reduces the chance of a reconciliation payback. You can update income again mid-year if circumstances change.
3. Report life changes. Marriage, divorce, new baby, child aging off, change in immigration status, change in employer coverage offer. The marketplace uses these to validate eligibility, not just subsidy size.
4. Review all plans at your preferred tier. Filter by metal tier (Silver, Gold, Bronze) and sort by premium after subsidy. The lowest post-subsidy premium is not always the best deal — a $0 Bronze with a $9,200 deductible is wrong for a household with chronic conditions, while a $45 Silver with cost-sharing reductions is often the right answer under 250 percent of FPL.
5. Check CSR eligibility if your income is at or below 250% of FPL. Cost-sharing reductions are only available on Silver plans and only apply if you actively select Silver. Letting the system auto-renew you into a Gold or Bronze forfeits them.
6. Verify your doctor on the 2027 provider directory. Use the directory linked from inside HealthCare.gov for the specific plan year, not the issuer's general provider lookup. 2026 directory and 2027 directory are different documents.
7. Verify prescriptions on the 2027 formulary. Pull the specific 2027 formulary PDF or searchable tool. Confirm tier and any prior authorization or step therapy requirements. A one-tier shift (Tier 2 to Tier 3) can change a copay by $20 to $60 a month on a chronic medication.
8. Submit. You get a confirmation number. Save it. The issuer will send a separate welcome packet in late December or early January.

Your plan is being discontinued and the crosswalk is wrong

If the renewal notice says your plan is being discontinued and the crosswalked plan has a different network, go into active selection and pick a plan whose network actually includes your doctor. The crosswalk is a default, not a decision — you are free to pick anything available in your rating area.

You got married, divorced, or had a baby in 2026

Household size changes the FPL calculation and often changes the applicable-percentage table result. Any household event that was not reported during the year must be reported during Open Enrollment, or auto-reenrollment uses stale household data.

Your income went up significantly

If your 2027 projected income will exceed 400 percent of FPL and the subsidy cliff has restored for your situation, the entire APTC may disappear. Model the number before Open Enrollment ends so you are not surprised by a full-price January premium.

You gained employer coverage mid-year

Enrollment in an affordable employer plan (as defined by the ACA affordability rules for 2027) ends your eligibility for marketplace APTC. If you let auto-reenrollment run, you will owe back the January APTC at tax time. Terminate the marketplace plan with an effective date on the day employer coverage starts.

You are turning 65 in 2027

If your 65th birthday falls in 2027, you have a Medicare Initial Enrollment Period that begins three months before the birth month. Coordinate the end of the ACA plan with the start of Medicare to avoid a double-premium month. Do not let auto-reenrollment carry you past the Medicare start date.

Open Enrollment keeps going through January 15, 2027. Changes made December 16 through January 15 take effect February 1, not January 1. The January 1 coverage you were auto-enrolled into stays in force (and bills) for the month of January; a change you make in early January takes effect the following month. The practical consequences:

• You still owe January premium at the auto-reenrolled rate. Non-payment of the first month triggers a grace period and eventual termination, but if you want continuous coverage you pay the January invoice even if you plan to switch for February.
• Your January deductible accumulation starts on the auto-enrolled plan. If you switch for February, the deductible generally restarts under the new plan. In practice this matters only if you had a January medical event; otherwise the lost accumulation is small.
• CSR eligibility only kicks in when you actively select Silver. If auto-reenrollment moved you off Silver, you forfeit CSR for January even if you move back to Silver for February.

After January 15, 2027 the Open Enrollment Period closes. To change plans you need a Special Enrollment Period event (job loss, marriage, birth, move, loss of other coverage, income change crossing a key threshold). The low-income monthly SEP for households under 150 percent of FPL remains in effect in 2026-27 and does not require a life event, but it only applies at those income levels.

Auto-reenrollment is useful when nothing has changed and the plan is still well-matched to your household. In 2026 — with benchmark Silver churn, formulary rework around GLP-1s and newly negotiated drugs, and a continued reshaping of the applicable-percentage table — the odds that doing nothing produces the right 2027 outcome are lower than usual. The fix is a 30-minute active selection in the HealthCare.gov portal (or your state exchange) by December 15, 2026, with current income, current household, and a quick verification of your doctor and prescriptions against the 2027 directory and formulary.

The deadline is unforgiving. Twelve-oh-one on December 16 is already auto-reenrollment time. Set a reminder for the first week of December so there is still buffer to re-check the numbers and submit a clean application.