This has happened to me a few times and it has always been easily corrected. In fact if you look at the thread announcing this year's cat changes I even posted a warning about checking reservations for this...

The explanation has usually been there was a back of the house reason to update the reservation, like to give you an UG, and the reservation flipped to the new points total. I feel bad for the people that do not notice or that do not know all you have to do is point this out, and it should not still be happening at least fifteen years after the first time it happened to me, and when it happens to you of course you are going to be concerned/upset, but it winds up being a nothing. The first time this happened to me it was the Niagara Falls Marriott and I called in a panic. The poor woman who answered the phone couldn't get me to a manager fast enough, but the manager explained she had given me an UG, changed my reservation, triggered the charge and corrected it quickly.