Boy, this really is annoying! I've had similar happen to me in refund situations (where the refund was approved, and after I sent my paper ticket in, it was returned to me with a note that said, "You can't get a refund." We had to sort through the history and go through hoops to get things the right way again).

It's cases like this during which I wish UA would call the customer before changing anything. Perhaps this is asking for too much, as I am sure that the majority of mail they get is from uninformed passengers rather than knowledgable ones.