A couple of things that I would start with:

Is it a Corporate owned or Franchised hotel? If its Corp, you shouldn't have a problem going through the Hh program as long as they see the supporting documents.

Are you planning on staying there much longer, do you have the ability to say you are going to stay there longer whether you intend to or not and tell them that you will be moving immediately to their nearest competitor. If you have collegues that stay there, notify them and try to influence them to change hotels, let the current management and cc Hh on the notice.

Write a letter to the CEO of Hilton. don't laugh this works, at least as far as having someone get back to you.

Notify the hotel that you will go public (internet) with the issue of changing the terms midstream. Even if you are just posting that here. IMHO, you should tell us the name of this hotel so we can avoid it. I wouldn't stay at this hotel knowing that they would do this.

Maybe you have done this, but go to the GM of the hotel, or the regional manager if applicable.

If the former management is still at hilton, see if you can contact them.

Small claims court. This seems like a valid contract to me (I'm not a lawyer but if its in writing it should hold)

I am assuming that this is as clear cut as it appears in your thread.

Hope you get your points