Answer is....it depends. I wouldn't assume they will accomodate you.
It depends on how full the Club and Lab are.
It depends on if your nine year old moves down (where she will be bored) or your seven year old is mature enough to move up. (They seem to be more lienient on moving down than up."
It used to be the "rule" was "up within 30 days of the next age group, down not a problem" (as long as you weren't talking about ridiculous difference in age - a twelve year old wanting to hang with three year olds). But that doesn't seem to be the case all the time and some people have been able to move kids up that have birthdays six months away.
They also seem to want to move down once only - people have been told that once their eight year old moves to five-seven, they won't move him back regardless of how bored they are. And they encourage you to put your children in age appropriate groups.
The 5-7 groups "home" is the Club - where the 3-4 year olds are - but they spend time in the Lab - with the 8-9 and 10-12 year olds. I don't know how much a nine year old would enjoy free play with three year olds.