When will I get my refund for premiums paid to my old Medicare plan after I started a new plan? Who pays the refund?
If the Social Security Administration (SSA) withheld the Medicare plan premium, Social Security will refund your premium. You should get this refund as an individual payment, separate from your regular monthly benefit, within six weeks after enrolling in a new Medicare Part D or Medicare Advantage plan.
Please note - it is possible that the SSA will need more time to refund your over-paid plan premiums. We have heard of a situation where the SSA refund did not occur until October (for an over-payment made in January).
(basic information from: U.S. Department of Health & Human Services)