I'm in the same boat. You do not have to pay the Tricare premiums. You do have to pay Medicare Part B premiums, which Social Security automatically deducts from your monthly SSDI payment. If you are still paying Tricare Prime premiums after you are enrolled in Medicare Part B premiums, call Tricare and they will adjust it. They cut my family premiums in half, so my wife still has Tricare Prime while I am using the VA and Medicare. Also, Tricare acts as your medigap plan. Whatever Medicare doesn't pay, Tricare will cover the rest of the cost.