Crappy situation, sorry to hear. One thing to check, have rates gone up at the property since you booked your suite? I find it not so likely that a property would overbook the top suite, kind of like airlines are careful with overbooking First.

Either they can get more money for it now or need it for something like a wedding party or such that will generate much higher revenue than you will.

Back to the policy, it only works of you are walked. So no, I don't think it will help you here. Best approach may be to negotiate with the property. What would a downgrade look like, e.g. a different lower level suite or normal room etc ?

Let them know you got a special event too and ask what they can offer to make it worth your while to consider the downgrade, e.g. (depends on property and what facilities it has) dinning credit, spa credit, other waived fees (parking, ..), extra points or a much reduced rate or if things like 2 nights paid you only pay 1 night at original rate. You get the idea.

If there is stuff that you would like that the property can give to you easily (stuff under their control) it may work best. I'd not threaten, but try to be "cautiously collaborative / sympathetic", but assert your rights as well. If the hotel is being stubborn, cheap, stupid then you can always tell them that you will escalate to corporate IHG and open a case.

It may not change much, but hotels also don't like the scrutiny that comes with such a complaint. Asking the hotel to book you an equivalent suite in another property will be very tough to accomplish unless the company that owns the hotel (not the chain brand on the door) owns another similar hotel that you like and they are open to work something out there. Along those lines. Good luck.