Medicare open enrollment: the one decision that doesn't come back
Medicare open enrollment runs from October 15 to December 7, and anything you change takes effect on January 1. Most of what it covers is an annual admin job: comparing drug plans, checking whether your plan still covers your prescriptions, moving between Medicare Advantage plans.
One of the changes on the list behaves differently from the rest. Moving from a Medicare Advantage plan back to Original Medicare is simple to do in this window and can be very hard to undo later, for a reason that has nothing to do with open enrollment at all.
What open enrollment actually covers
Between October 15 and December 7 you can:
- Move from Original Medicare to a Medicare Advantage plan, or back again.
- Switch from one Medicare Advantage plan to another.
- Join a Medicare drug plan, switch to a different one, or drop drug coverage.
- Move between Medicare Advantage plans that include drug coverage and ones that don't.
Every item on that list is a change to coverage you already have. Your plan has to receive the request by December 7, and the new arrangement starts on January 1.
Is this when you first sign up for Medicare?
No. First-time enrollment in Part A and Part B happens in your own seven-month window around turning 65: the three months before the month you turn 65, that month, and the three months after. That window is personal to your birthday and has nothing to do with October.
The distinction matters because the penalties attach to the wrong one. Missing your initial window adds 10% to the Part B premium for each year of delay, and it stays for as long as you hold Part B. Nothing in October carries a penalty like that. Somebody still covered by an employer plan at 65 generally has a Special Enrollment Period instead and owes nothing, which is worth checking against your own coverage rather than assumed either way.
Why is leaving Medicare Advantage close to one-way?
Original Medicare has no cap on what you pay out of pocket. Part B covers 80% of approved costs and the remaining 20% has no ceiling, which is what Medigap policies exist to cover.
Medigap has its own enrollment window, and it is not this one. Under federal law it is a one-time six-month period that starts the first month you are 65 and enrolled in Part B. During it an insurer cannot refuse to sell you any policy it offers, cannot use medical underwriting, and cannot charge you more for your health history. It does not repeat.
After it ends, an insurer does not have to sell you a Medigap policy at all unless you have a specific guaranteed-issue right. Outside those situations they can underwrite, price on your health history, or decline.
So the shape of it is this: somebody who chose Medicare Advantage at 65 and wants Original Medicare at 72 can make that switch in any October. Buying the Medigap policy that caps their costs afterwards is a separate question, answered by an insurer who is allowed to say no. The move out is open every year. The thing that makes the move survivable may not be.
What changes every year even if you do nothing?
Your plan does. Drug formularies, premiums, deductibles, pharmacy networks and provider networks are all set annually, and a plan that fitted last year is not guaranteed to fit this one. Plans send a notice of what is changing before open enrollment opens.
Doing nothing is a choice that re-enrolls you in next year's version of your plan, not this year's.
What this doesn't model
MangoDime estimates Medicare as a cost: Part B and Part D base premiums from 65, plus any IRMAA surcharge your income triggers. It does not model which plan you hold. Original Medicare, Medicare Advantage and Medigap are not separate options in a plan here, so nothing in your figures changes based on the decision this article is about.
It does track your own initial enrollment window and will tell you when it opens, which is the deadline with the permanent penalty behind it. IRMAA is applied against the same year's income rather than the two-year lookback the real rule uses.
Where to check your own
Your plan's annual change notice lists what is different next year. The Medicare Plan Finder at medicare.gov compares drug plans against the prescriptions you actually take, which is the comparison that usually moves money. Your State Health Insurance Assistance Program gives free counseling, and for the Medigap question specifically, an insurer can tell you whether they would underwrite you before you give anything up.
Dates and rules here were read from medicare.gov on October 7, 2026. This page is reviewed each January.
Last updated October 7, 2026 · MangoDime