Agreed, only Reservations will help. Remind them that they should be calculating the fare difference based on fares that were in effect at the time of the original ticket purchase.

I doubt that moving from a 2- to 3- week trip meant breaking a max stay requirement.

Only difference I can think of would be a day of week restriction, but that should be small.

OP - what date did you purchase your ticket? Someone here can look up the different M fares available on that day.