Generally, if you join a Medicare Advantage Plan or Medicare Prescription Drug Plan, you can only change plans from October 15 through December 7 of every year.
This change will be effective January 1 of the following year.
If you are also eligible for Medicaid, you can switch plans at anytime.
(Primary Source: Centers for Medicare and Medicaid Services - Medicare and You Handbook. This content may have been enhanced by Q1Group LLC to include further examples, explanations, and links.)
Medicare Supplements fill the gaps in your Original Medicare