If you have a Medicare Supplement plan (also knows as Medigap), then you can switch plans anytime of year. Many people mistakenly believe they can only switch Medigap plans or carriers during the Annual Election Period (from October 15 to December 7 each year).
The Annual Enrollment Period (AEP) is the time of year to switch (if it makes sense) to a new Prescription Drug Plan or Medicare Advantage plan.