The short answer is, sometimes.
During the AEP (Annual Election Period between Oct 15th and Dec 7th each year), you can change from one Medicare Advantage plan to another Medicare Advantage plan or from one Part D Drug to another Part D Drug plan. BUT….you can’t always go from a Medicare Advantage Plan to a Medicare Supplement Plan (AKA Medigap).
In most cases (unless you are losing credible coverage through an employer or your Medicare Advantage doesn’t renew it’s annual contract with Medicare) you have a ONCE IN A LIFETIME 6 MONTHS from when you are new to Medicare Open Enrollment to sign-up for a Medicare Supplement Plan. (If you are new to Medicare, you can try Medicare Advantage for one year maximum as well and then get a Supplement). If the 6 months is up, all bet’s are off in most cases. Then you must answer several health questions and you could be on the outside looking in for the rest of your life.