I called Frontier and was told that in order to upgrade I would have to pay a $100 change fee plus any fare difference. Since the base price of the ticket is about $50 more than when I purchased and the Classic pass costs $50 more, the net result is $200 extra after the change fee is added in. Not worth it. One person in our party did purchase a ticket via the Frontier website. Looks like they are now traveling with 2 - 49.9-lb bags.
Thanks for the info. I guess we will have to decide if bringing our skis vs renting is worth it with respect to cost and convenience.