I was booked to fly home overnight, MEX-YYZ-YUL-YHZ, at 0010, booked in P; Wanted to change to either the 0845 or 1345. Reservations quoted me a $1100 fare difference + $100 scam fee to change.
A relatively quick (it is AC, the concierge needed help from someone) got me the change for free (into C).