Well I guess it would depend how put out you'd be if you didn't have a place? Can you talk with your director and find out if she can give you any more idea of what is likely to happen?
Although he might be in a younger room in the other centre, all centres must provide a program for each child according to their individual developmental level, regardless of what age/level other children are at. You could ask the other centre about the ways they would cater to his needs - what kind of equipment would be available to someone older, what activities would they plan for him, etc. There could actually be some advantages eg. being the oldest, he may be given more opportunities to help carers and be independent which is great way to stimulate and encourage. Also, if many of the children are having two sleeps and your DS is down to one, he would probably have more time with the carers than you might imagine during those times when most are asleep.
You mention that you are happy with his current carers - that's so important. How do you feel about the staff at the new centre? Do they make you feel as comfortable?

It's a tough position to be in for sure. I can't say which way to go, but I hope you are able to come to a decision you're happy with. Sorry I don't really have an answer for you. Good luck!