IME, I've never had a problem canceling, even if it was after the deadline, if I was switching to another Marriott property. I've done that many times. YMMV.
Thank you for mentioning this, OP. I've got 17 upcoming reservations in the queue and just double checked: many of them do have cancellations that are earlier than the day of check-in. Eight of the 17 are one, two or three days in advance. This is different than in years past. Some hotels did have earlier cut-offs, but not this many.