When you turn 65 or enroll in Medicare Part B for the first time (whichever comes later) you become eligible for your Medicare Open Enrollment Period. This means that any and all Medicare plans must accept you regardless of previous health status. This period begins 3-6 months before your 65th birth month or Part B enrollment month (if you are delaying enrollment into Part B), and continues for 3-6 months after your Part B effective date.
In 2015, Congress passes MACRA and, among its provisions, it eliminates the ability for new Medicare beneficiaries to enroll in popular Medicare Supplement Plan F after December 31st, 2019.
Medicare Supplement (otherwise known as Medigap) plans are guaranteed renewable for life. No requirement to change each year.
Medicare Advantage and Part D Plans are offered on a year-to-year basis and renew each year on January 1st. The Annual Enrollment Period starts on October 15th and runs through December 7th. Any plan changes you make during this period become effective on January 1st.
Per government guidelines, Medicare Supplement (Medigap) Plans must be standardized. This means that every company must offer the same exact benefits. All standard Medicare Supplement plans are accepted at any doctor that accepts Medicare NATIONWIDE.
The comparison between companies for the same letter plan (A,B,C,D,F,G.K,L,M,N) is apples-to-apples and the only difference is the monthly premium you pay each month.
Plus, as long as you continue to pay your premium, you can never lose your coverage.