When can I switch from traditional Medicare to a Medicare Advantage plan, or go back?

Medicare.gov says you can only join, switch or drop a Medicare Advantage Plan at certain times, called enrollment periods [1]. Here are the main ones.

Open Enrollment, October 15 to December 7. You can join, drop or switch to another Medicare Advantage Plan. You can also switch from Original Medicare to a Medicare Advantage Plan, or back. Coverage starts January 1 of the next year. The plan must get your request by December 7 [1].

Medicare Advantage Open Enrollment, January 1 to March 31. This is only if you are already in a Medicare Advantage Plan. You can switch to another Advantage plan. You can also drop your plan and return to Original Medicare, and then join a separate drug plan [1].

A Special Enrollment Period is only for certain life events. Examples are moving to a new address, losing or changing your coverage, getting Medicaid, or getting Extra Help to pay drug costs [1]. The dates differ in each case.

The Medicare.gov page lists more enrollment periods and the exact coverage start for each. Medigap rules are separate — "Don't know yet" from this page.