Military personnel can maintain TRICARE coverage if they decide to enroll in a Medicare plan as well. Having the military benefits in place prior to retirement means you simply need to add Medicare. Once you become eligible for Medicare, you'll have 90 days to decide if you prefer TRICARE. TRICARE is a program overseen by the Defense Health Agency (DHA), serving millions of active and retired military employees. Your military status determines what your plan covers. It's possible for Medicare to fill gaps that your TRICARE plan doesn't include.