I'm with Sharon and Mr. Vker. You don't get to change the rules after you have accepted the reservation. If it was a special event, those details should have been loaded in when those dates elevated to the reservation system.

I would definitely respond and indicate that you expect the reservation to be honored and that you do not authorize them charging you in advance. I would also advise that if you arrive and your reservation is not honored you will invoke the Ultimate Reservation Guarantee and demand the compensation you are entitled to.

If you have further issues with the GM I would contract Plat customer service and see if they can educate him.