Well, I had the opposite experience.

Booked SFO-ORD on a 777 so I could get 2 sets of window/aisle seats together for myself, wife and kids.

They later switched the equip to 747 and UA was "kind" enough to let me keep my original seats.. Only problem is that the 3-4-3 on a 747 meant I got window/middle instead. Geez, next time don't do me any favors.