Downgauged due to unplanned maintenance on 763.
A day-of downgauge like this wouldn't happen due to light loads as there are crewing implications - the flight crew that was scheduled to fly the 763 can't fly the 321, so new pilots had to be drafted.
FYI, in the past 90 days AC791/792 has only been downgauged 4 times to a 321, so it is fairly reliably operated as a 763.