'Well, wouldn't you rather go out with a hell of a party? We can drink ourselves silly while Rome burns.'
OK, maybe not.
I wouldn't let them get completely away with it, though, by saying it is OK. It's not OK - they have cost you additional expense by doing this after you have given final numbers to the caterer! In fact, I'd want to say something to that effect but I'm not sure it would be polite.
Three weeks out seems like a long lead time. Could you try calling the caterers and see if you can change your numbers?