I don't have first hand knowledge either, but on this board the topic of putting kids in different levels has come up and it seems like going 'down' is easier than going 'up' in level.
In September, my twins will be celebrating their 10th birthday on a 3 day cruise. The day we board, they are 9; the next day they turn 10. I am going to put them in the 8/9 group (hoping there won't be a problem because the ARE 9 on the day we board). My girls just aren't ready to be with the 12 year olds (and maybe almost 13 year olds). They are still little girls and like to play like little girls (dolls, stuffed animals, etc.).
Next year, for their 11th birthday they can go in the next level up (10-12 year olds), but this year I don't want them to move up yet. So I am hoping there won't be any problem.
I hope your situation works out for you too!
DJ