You must have Parts A&B to ENROLL into a Medicare Advantage Plan. Then you have a 7 month window during the Initial Coverage Period when you first turn 65 to join a Medicare Advantage Plan. (3 months before, the month of, and 3 months after your birth month).
Or - From October 15 - December 7 each year (Open Enrollment).
Or - For a Special Enrollment situation such as losing coverage at work.
January 1 - February 14 each year is the Medicare Dis-enrollment Period. You can leave your Medicare Advantage plan and change to ORIGINAL MEDICARE with or without also selecting a separate stand-alone Medicare Drug plan.
You can't make any changes to your coverage during this period if you have original medicare already. You can't change from one Medicare Advantage plan to another or from one Part D prescription plan to another during this time.