Medicare's annual enrollment period runs from the middle of October to the first week of December, and the advertising that fills those seven weeks describes a market you can move around in freely. Within Medicare Advantage that is roughly true. Moving the other way, from a Medicare Advantage plan back to Original Medicare with a supplement policy attached, is governed by a different rule that the advertising does not mention, because it is not a Medicare rule at all. It is state insurance law.
The federal guarantee that an insurer must sell you a Medigap policy regardless of your health lasts six months. It begins the first day of the month in which you are both 65 and enrolled in Part B, and it does not repeat. Medicare's own guidance says so plainly: after that window, there is no federal guarantee that any insurance company will sell you a Medigap policy at all.
What the Six Months Buy
Inside the window an insurer cannot refuse to sell any policy it offers, cannot use medical underwriting, and cannot charge more because of a pre-existing condition. Outside it, in most states, all three become available to the company. It can decline the application, it can price on health, and it can impose a waiting period.
Of the 64.2 million people with both Part A and Part B this year, about 35.2 million are in Medicare Advantage, and roughly 90 percent of those aged 65 and over have no guaranteed issue protection beyond the one-time trial right. About 12 million people in traditional Medicare hold a Medigap policy; about 3.5 million hold no supplemental coverage at all.
The Mechanics of a Door That Opens Once
The structural consequence is a filter that sorts people by health at exactly the wrong moment. Federal auditors found that Medicare Advantage enrollees in the last year of life left for fee-for-service Medicare at more than twice the rate of everyone else. And Medicare spent 27 percent more on people who had disenrolled from Medicare Advantage than on comparable people who had been in traditional Medicare all along, a gap that widened to 46 percent among those 85 and older.
That is the population trying to buy a supplement policy years after their one window closed, in a market where the insurer may now ask about their health. Nobody publishes how many of them are turned down. The transaction happens between an individual and a carrier under state regulation, and no federal agency counts it.
The Exceptions, and Where You Live
There are real exceptions, worth knowing before the enrollment period rather than during it. Federal law creates guaranteed issue rights in specific situations, each with a window of 63 days after coverage ends: a Medicare Advantage plan that terminates or leaves your area, a move out of its service area, employer or union retiree coverage ending, and a trial right if you joined Medicare Advantage when first eligible at 65 and want out within twelve months.
The first of those matters more this year than usual. About 2.9 million Medicare Advantage enrollees, roughly one in ten of those in individual HMO and PPO plans, were pushed out of terminating plans for 2026, against about one percent a year from 2018 through 2024. A terminating plan is a listed federal trigger, so a large group received a guaranteed issue right as a side effect of losing a plan they did not choose to leave.
State law does the rest. Connecticut and New York guarantee issue year-round and community-rate their premiums. Massachusetts and Maine run annual windows. Roughly eighteen other states have a birthday or anniversary rule allowing a switch to a policy of equal or lesser benefits within a set number of days each year, and several of those rules are new: Delaware, Indiana, Virginia, Utah, Rhode Island and West Virginia all took effect in 2025 or 2026, and New Mexico's begins in 2027.
What the Calendar Does Not Say
Price follows the same logic. A community-rated policy charges everyone the same, an issue-age policy locks the rate to your age at purchase, and an attained-age policy rises every year simply because you are older. Medicare's own illustration of the third is 120 dollars a month at 65, 126 at 66 and 132 at 67, before any inflation increase.
The annual enrollment period lets you leave Medicare Advantage any year you like, and says nothing at all about whether anyone will sell you the supplement policy that makes leaving affordable. Two questions answer most of it, and both can be settled well before October. Does your state have a birthday rule or a continuous guaranteed issue right, which the state insurance department will tell you, and has anything happened in the last 63 days that opens a federal window, since a terminating plan, a move or the end of retiree coverage each starts a clock nobody sends a reminder about.
— John Stone