This is what AF did : cancel a YUL-CDG flight and use the a/c to fly YUL-Goose-ATL.
The problem again is the number of pax. Even the biggest a/c in AF fleet (apart from 380) the 77W in leisure configuration - by chance operating the late flight from YUL - was not big enough for 500 pax. So, an additional 737 had to be chartered.