It is not quite that simple. Yes you can change your advantage plan during the fall open enrollment. But switching supplement plans depends on many things. Here in Missouri we can switch to a different supplement of same coverage for a lower price during our anniversary date without going thru medical underwriting, but most states do not allow that. To switch from Advantage to supplement you will have to go thru medical underwriting. This is a very important point. If you are not Healthy and cannot answer no to the medical underwriting questions, you will NOT be eligible for a supplement, like the one time in your life, the 3 months prior to your birth month at age 65 , your birth month and 3 months post your birth month at age 65 upon your initial enrollment. This is the one most important 7 month period of your life where the decision you make COULD affect the rest of your life as pertains to health. If you remain healthy then you have no worries.
What happens is people fall ill and having been on an advantage plan and having hospital stays etc. They get tired of paying out of pocket up to their yearly maximum and decide a supplement plan looks good. Sorry Charlie, ain't gonna happen. You will be stuck in Advantage plan world for the rest of your days. Advantage plan world is not all bad, the monthly premiums are either very low or $0 and the copays are reasonable on most of them. If your remain reasonably healthy, you are going to come out ahead when the final tally is taken. The problem comes when you are diagnosed with "something",, then it can get dicey and most then wish they had taken the supplement when they had that golden opportunity to take it at age 65.
For those that are ill at 65 or know they will soon be ill then it is a no brainer, you choose a good supplement from a reputable company (A rated) and never look back, and don't be lured by the siren song of Advantage plans. You can be near death at age 65 and as long as you can sign the paper or have your POA sign it for you, then you are eligible for a supplement. You cannot be refused by the insurance company. The only reason you can be dropped no matter how ill you become is because you failed to make the monthly payment. This is why I advise everyone to always sign up for automatic bank draft from your account. You DO NOT want to forget to make a payment.