If you already have a Medicare Supplement plan that you wish to replace, you can apply for another plan. So there will be no gap in your coverage, just be sure not to cancel your current policy until you've received a new Medicare Supplement policy and have had a chance to review it. Also, if you have a Medigap plan issued prior to 2006, it may include prescription drug coverage in which case, by switching plan you will also have to select a plan under Part D of Medicare.