Medicare open enrollment runs October 15 through December 7, and for 2027 the arithmetic is unusually hostile to standing still. It is also unusually hostile to moving — because of a rule almost nobody reads.
The Setup: 2027 Pushes Everyone to Switch
CMS set the 2027 Part D parameters in its April 6, 2026 Rate Announcement: the standard deductible rises from $615 to $700, and the annual out-of-pocket threshold rises from $2,100 to $2,400.
The plan landscape shrank at the same time. KFF's analysis of the 2027 offerings counts 316 stand-alone prescription drug plans from 15 organizations, down from 360 plans from 17 organizations in 2026 — an average of nine choices per beneficiary instead of eleven. CMS projects the average monthly PDP premium rising only about $1, from $35 to $36, but that average conceals enormous dispersion. KFF found that United's AARP Medicare Rx Preferred plan is raising premiums by $50 or more in all 50 states and the District of Columbia, and by more than $75 in 37 of them.
Most consequentially: there will be no zero-premium stand-alone drug plans in 2027 for enrollees who don't receive low-income subsidies. Roughly four million people who paid $0 in 2026 will pay something next year.
That is a strong case for shopping your plan. Which brings us to the trap.
The Trap: Disenrollment Ends Your Smoothing
The Medicare Prescription Payment Plan — the program that spreads your out-of-pocket drug costs into level monthly payments at no interest — now renews itself automatically. Under 42 CFR 423.137(d)(10)(iv), your plan sends a Notice of Participation Renewal after open enrollment closes but before the year ends, and your participation carries into 2027 unless you opt out.
It carries forward only if you stay. CMS's final Part Two guidance is explicit: when an enrollee "disenrolls from the Part D plan, such as when switching plans during the coverage year or for a subsequent coverage year, their participation in the Medicare Prescription Payment Plan, as administered by the Part D plan losing the enrollee, effectively ends."
You will get a notice — CMS requires the losing plan to send a Notice of Voluntary Removal explaining that you can rejoin and how. But rejoining is an affirmative act you must take with the new plan. Nobody does it for you.
Why January Hurts Twice
Two bills arrive at once. Your old plan must keep billing the balance you still owe from 2026 in monthly installments — it cannot demand immediate full repayment, and cannot charge interest or fees. Meanwhile your 2027 drug costs are due in cash at the pharmacy counter until you opt in with your new carrier.
Timing compounds it. The maximum monthly cap equals the annual out-of-pocket threshold, minus costs already incurred, divided by the months remaining. Opt in during January 2027 and the cap is $200 ($2,400 ÷ 12). Wait until April and you are dividing by nine.
Take-up is thin enough that most people won't notice the gap until it bites: Milliman found participation at just 0.6% of all Part D beneficiaries as of July 2025, and 6.7% of non-subsidy enrollees who filled a specialty drug. Note too that the point-of-sale notification trigger is a static $600 — below 2027's $700 deductible, so a single large fill can set it off.
Practical takeaways: If you use the payment plan, decide first whether you're switching. If you are, call the new plan's number on day one of coverage and opt in again — don't wait for a bill. If you're staying, read the renewal notice rather than discarding it. And budget for the overlap month when the old balance and new counter costs land together.
Sources: CMS, Announcement of CY 2027 Medicare Advantage Capitation Rates and Part C and Part D Payment Policies (April 6, 2026); CMS, Final CY 2025 Medicare Prescription Payment Plan Part Two Guidance; CMS, Medicare Prescription Payment Plan: Update on 2026 Model Documents; 42 CFR 423.137(d)(10)(iv); KFF, "Many Medicare Part D Stand-Alone Drug Plan Enrollees Will See Modest Premium Increases for 2027"; Milliman, "The Medicare Prescription Payment Plan: Implementation in 2025 and implications for 2026."