I agree this is very poor form for Celebrity and that minimal OBC (assuming in the couple hundred $$) range which they always state as a "One time Good Will Gesture" would not be adequate after committing to a $5k+ upgrade.
I had something a little bit similar that was handled much better. I had booked an AQ room about 3 weeks before sailing. I got a call about a week later that they had oversold AQ and would I accept a CC stateroom with a refund of about 25%. I was concerned that the refund would be in OBC, but no it was refunded to my CC. Long story short I was able to pick any CC cabin I wanted. We had all the AQ amenities in our room and were able to eat in Blu. It was a win-win and was left satisfied. The exact opposite experience of the OP.
So in my opinion a better response from Celebrity would be to provide some of the PH amenities for the OP in their CS, like unlimited specialty dining, laundry etc. Maybe if the onboard staff is aware of what happened there might be some accommodation