I just completed a reward stay last weekend at Hilton Prague which was a Cat 3 before June 1st. I made my reservation on May 27th on HH website for the weekend of July 14th and the reward certificate was issued the same day. Due to personal reasons, I had to revise my reservation to one week later(i.e to July 21st) than the original reservation. On July 13th, I called up HH customer service and they said no problem in using the Cat 3 certificate (since the award was issued before June 1st) and changed the stay date to 7/21.

I emailed the hotel to confirm the same and they responded back to me saying that it would not be an issue using the Cat 3 certificate and they will honor it.
At checkin they asked for the award certificate and I gave them the Cat 3 certificate and no questions were asked.

BTW, Hilton Prague was exactly if not better as described in FT forums. Excellent customer service and awsome Executive lounge. This hotel met all of expectations. Highly recommended.