Returning from holidays in Europe on the 2nd Jan I had GOT-BRU-YUL-YOW with a 6hr stopover (AC8013 at 6:10pm orignally) in YUL booked due to unmarried segments when booking (PY lowest award).
Asked to SDC the YUL-YOW leg to either of the 2 earlier flights to YOW and was quoted a 100CAD change fee to the 4pm flight and that they coudn't put me on standby for the 2pm flight (which was the one I wanted to get on) at the service desk. I said 'no thanks' and called the concierge who put me on the 4pm flight free of charge and standby for the 2pm one (which did clear!).

No possiblitiy to do it in the app (likely due to checking being with Brusell Airlines).