Have had this a few times. Always my mistake as well I should add.

When I've approached Ambassador center, the opening is usually the same .... 'it's not allowed sir'. But in fairness with a little polite and firm coms, Ambassador will concede and flex.

In fact, I was told by AC that they are allowed one change or concessionary cancel per year. I rather got the impression that was for those with track record and volume of stays but I could be wrong. I've made a couple of howling bad and large reservations on non flex which have proven to be way off target and they have been accommodating.

I do think in the main that it depends on the degree of humility, polite firmness and quality of communication you strike up. You should get a good hearing. I would not bother dealing with the hotel personally.