All Medicare beneficiaries can switch plans once a year. This happens during Annual Enrollment Period or AEP. Medicare’s open enrollment runs between October 15 and December 7. Any changes will take effect on January 1.
During that time, you can switch Medicare Advantage plans, change from Medicare Advantage to Original Medicare, swap Original Medicare for Medicare Advantage or change Part D plans.
There's another open enrollment period from January 1 to March 31, but it's limited to only switching Medicare Advantage plans or moving from Medicare Advantage to Original Medicare. Only one change can be made during this period.