I had a slightly similar issue recently... in my case I had changed my reservation, got an email from Marriott saying that too.

However, a few days later I happened to check the Marriott website (on the evening of the changed reservation - too late to cancel) and saw that my original dates had been restored. (It was only a day difference due to a public holiday in the UK)

However, I didn't have to take it any further as I'm a regular at the hotel in question - so I rang them up and asked them nicely and they just shunted my stay to start a day later.

However, it did make me VERY wary of any reservation changes now - so, I've got into the habit of just checking the upcoming reservations tab when I've made any changes to see if this is the case.

Also, FYI, I'd rung up the Marriott Platinum line about this case and they said that looking at the reservation history I'd tried to change the reservation - but there was 'no availability at the rate I'd chosen for those days' - so that was why the change hadn't come through. I was pretty sure it wouldn't be a problem if the hotel had been fussy since I had the email confirmation saying I'd had the dates changed and its not my fault that the Marriott website looks to have a minor bug in it!