Medicare Advantage plans are a type of Medicare health plan offered by a private company that contracts with Medicare. They require annual renewals, enrollments and disenrollments.
You still have Medicare if you enroll in a Medicare Advantage Plan. This means that you will still owe a monthly Part B premium (and your Part A premium, if you have one). Each Medicare Advantage Plan must provide all Part A and Part B services covered by Original Medicare, but can do so with different rules, costs, and restrictions that can affect how and when you receive care. Medicare Advantage Plans can also provide Part D coverage.
To switch to a new Medicare Advantage Plan, simply join the plan you choose during one of the enrollment periods. The most common period is from October 15th thru December 7th of each year. You'll be disenrolled automatically from your old plan when your new plan's coverage begins.