I've run into this, too. One time, not very long ago, I had a reservation for three nights and couldn't arrive on the first night because of a flight cancellation. I called in plenty of time (before the cancellation deadline) and shortened the reservation to two nights (the last two of the original three nights). The airline took care of me the first night, in case you're wondering.

The daily rate went up considerably. I asked why. (This was not a case of needing weekend days or anything like that. The original reservation was for Friday-Sunday nights; the revised reservation was for Saturday-Sunday nights.)

Anyway, after multiple, long conversations with Marriott reservations and the hotel itself, I was eventually told by somebody that Marriott's practice in my situation is to cancel the original reservation and make a new one at the lowest rate then available, which is almost always higher.

I objected strenuously to this and insisted that I would pay only the rate that I had reserved. They were equally insistent that I was, in effect, cancelling that reservation and making a new one. In the end, the hotel manager forced my original rate into the system, but it was clear that this was a special favor for a Platinum guest who would not take no for an answer!

Bruce