I did it on Sunday at YYZ at the transit counter. It took about 20 minutes and 3 agents conferring with each other as nobody knew how to do it or if it was allowed.
For some reason they where only able to put me in Y (I had upgraded the original flight) but the GA was able to switch it back to J, which incidentally was not full with 3 empty seats.
Good luck finding someone at LGA knowing what to do.