Actually, I have had this happen with both WJ and AC in past years. WJ will fly the route, and they start offering tickets almost a year in advance. But they only balance their routes and fleets for operating efficiency a several weeks in advance. Note that adding a route somewhere else in the system can have ripple effects across the system, and impact many other scheduled flights; a single jet will often fly many routes in a single 24-hour period, sometimes with as many as 3 different crews manning the controls and cabins.
So yes, it happened in 2012, our evening direct flight from Ottawa became a mid-morning direct flight and that did not work for us. We routed through Toronto on an Evening flight instead. Wouldn't say the change was last minute; we knew I think around September for our March flight.
Our similar AC experience was with a flight from YUL, it departed a couple of hours later than what we originally booked; had we known sooner we might not have opted for overnight hotel in Montreal the night prior.
Flight changes with US airlines may be less frequent as the booking windows for tickets down there are much smaller. But then you have to wait much longer to purchase your tickets.
