Assuming you ordered a 7 free night award and it hasn't been used, then there shouldn't be any issue with getting your points back. Is this an e-cert or a paper cert? If it is a paper cert you will have to send it back to Marriott.

After you have returned your current award, you should be able to order a TP award with the airline miles of your choice. Actually, if you call MR, they can probably do the complete transfer to a TP award for you while you are on the phone.