So I had the misfortune of dealing with this about a month ago. Booked a multi-leg trip (MEX-PVR-JFK) in J. Had to change the PVR-JFK leg by one day. They wanted $15k MXN to do that.
Because I was in the 24 hour period, I cancelled the ticket. They only credited back a small portion of my points. (there were two legs, one at 79k and one at 49k. They only put back the 49, I am fighting about the 79k)
I was not charged anything that I can see..