Notations In Record?
 

I just got off the phone with a CSR and, for the second time, attempted to change a flight due to a UA schedule change that prevented me from completing a one day MR. I requested a change in my destination airport to one that was within 100 miles that would enable me to complete the same day MR. For the second time, I was denied the request. It dawned on me that maybe the decision reached in my first request was noted in a record. Do CSR's maintain a record of their decisions reached and is this information pulled up when a call is made on the affected itinerary? Am I now SOL?

Most, if not all, interactions on a PNR are noted in it. This is usually a good thing, as it can save time on subsequent calls. Whether that's what's causing your problem, or if it's just outside the rules, is unknown.