UA Mechanical Cancel

Experts out there, I had United cancel 22 hours in advance a 767 from IAD to DEN on Sat Dec 13 flt 923. Seems like UA could have found a replacement aircraft. Seat map showed fairly full flight. I couldn’t get any specifics from UA, any UA people got a guess on this?

The reason listed is at united cargo is “operations.” With the 763s getting premium cabin upgrades they may not have any nearby replacements.