Okay, this is weird, I thought it was just that the time changes hadn't been included in my reservation properly but it actually turns out that united.com and aircanada.com are showing two different times for the same flight.
The flight in question is UA 547 / AC 5141 on September 25th. UA shows it as leaving at 7:00, whereas AC shows it leaving at 7:10. My inbound flight, AC 156 is due to land at 5:53, so according to AC I have a 1:17 layover which is above MCT, but if the UA time is right, my layover is only 1:07, below MCT.
However, since it is a AC ticket and they are showing a 1:17 layover, I bet they won't do a free change due to MCT.