Going from a Renaissance to a Hampton Inn is a serious downgrade. You should invoke the ultimate reservation guarantee. They should move you to an equivalent property. What they're doing is basically like sticking you in a Fairfield Inn.
Is the Ren paying for your stay at the Hampton, or are they expecting you to pay the same (probably higher) rate that you would've if at the Ren?
Try calling Marriott customer service again... even though its gone down the crapper. Maybe you'll get a better agent. Or if not ask to get escalated.
Fyi the ultimate reservation guarantee is:
"If for some reason we’re unable to honor your reservation, we’ll pay for your accommodations that night at a nearby hotel and compensate you for the inconvenience. To be eligible, you must provide your Marriott Rewards membership number when making a reservation."
For a Renaissance, that is $200 and 90k points.