That's exactly my concern. In our case, we made our reservations in advance specifically to lock in the Cat 7 rate on a few hotels that are going to Cat 8. If the rate changes on March 5th, our trip will jump from 430,000 points (which is achievable) to nearly 700,000 points...which would entirely kill our ability to take the trip. It makes me grateful I didn't book our flights yet. Think about the backlash from elite members who booked flights expecting to have that reservation be honored at the points price they booked at...
It's APPALLING that they'd change the price of reservations that have already been made. And with less than a month's notice on the change. That seems cruel, and (since those Cat 8 hotels are pricey) they're harming some of their most loyal customers too...
Yet another devaluation of Marriott's "reward" program...