The typical response with airlines is that they want to cancel and reissue the whole itinerary and when they do that you might get no seats at all, seats in first on the first flight and no more J on Air Canada so ask if they can check if there are seats on both legs available. Good luck with doing this on a call to them.
Do they actually price (award miles) Y and J on on itinerary or did you pay for J (F) for the whole route and all you got was Y and J in which case they should be more willing to change the Y to F because it was paid for as all J/F to begin with.