Hi David,
These are really sticky situations, especially since you don't have anything in writing. I've had something similar happen one time, and I went all the way to the GM of the hotel. After explaining everything, they did work with us.
Problems of counting on Hotel or event wedding coordinators, is that many times, you'll go through several before the actual date.
My recommendation would be to set up an appointment with the General Manager, ask the banquet manager to be there and have a discussion. Explain things exactly as you've told me here, that the room was promised, but never put into writing, and now the person you had originally been working with, has proceeded to have another event in the room that had been promised to you.
Unfortunately, without having it in writing they aren't under any obligation...but if they have good ethics then they should honor the agreement.
Do you have a coordinator that you've hired to assist you? It would also be helpful to have them involved as well. I'm sorry to hear you are going through this as it can really put a lot of stress on a couple.
Please let me know if I can be of any further assistance.
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