I spoke to MX yesterday concerning my flights ORD-PVR which were to have taken place on Nov 9 returning Nov 13. Something has come up and I won't be able to make those dates.
I can't ABSOLUTELY be certain, but I was told by MX that:
1) so long as changes to dates were made prior to departure of first outbound flight, and 2) availability was there,
I'd have NO problems. They mentioned that if I wanted to change dates after the departure of the outbound flight, a trip to the airport and a "re-validation" of the ticket would be required.
I'm keeping my fingers crossed that MX rep was right, as I won't be able to use my originally booked dates.
Best wishes.