You can keep your coverage and also sign up for Part D. Your insurance company must notify you if your plan provides creditable coverage — that is, if it is at least as good as Medicare’s. If it is not creditable and you don’t enroll in Part D when you are first eligible, you will be subject to a late enrollment penalty if you enroll in the future. If you enroll when you are first eligible, you can use your individual insurance to supplement Part D, but none of the payments made by your insurer count toward your Part D out-of-pocket cost limits.