It would probably be quicker to send the crew that is on standby in YYZ back to the hotel for rest and then later summon them back to the airfield to operate the delayed service, rather than rounding up a new crew from CDG and sending them over to operate a delayed service. AF will have crew hotels block-booked and arriving and departing at any time - even unscheduled at short notice - would not be an insurmountable issue.

I'm not even sure if company rules would allow a TATL crew to operate a service from YYZ almost immediately after arriving off another TATL from CDG as passengers; they would probably require a certain minimum rest period (even though they weren't "working" on the flight over) before being legally allowed to operate a TATL flight back to CDG, too.

This may also be why flights get renumbered; if the crew that are already at the airfield and ready to go can hang on there long enough to waiting out the delay and operate the planned flight - albeit delayed - without "timing out", then the same flight number can be used. If the delay is looking like it requires the crew to be sent off duty before operating the flight, then a new AF4*** number is used (which always happens when the flight departs past midnight, i.e. the next day)