If you're on the ATR, you will stay on the ATR for a while. The seat lock is something like a year, but they will bypass the ATR F/O CR7 transitions and give them to 200 guys instead. So, you cannot bid the CRJ200 or 700 in class and expect to get it anytime soon. They don't like to waste money retraining people without a necessity for it.
Now. That said. The plane is supposed to be gone in less than a year anyways. They seem to be pretty serious about it this time, not like the last 3 or 4 times they've said they'll get rid of it. If this happens, you can bid and probably get whatever you want.
The ATR is a cool plane, I don't know why there is a stigma against it. Actually I do understand why. It's a "loud prop," (actually it's a lot quieter than the Saab or EMB) but keep in mind that the plane is about as high tech as it can get without an FMS, will make you a better pilot than those flying the barbie jet, and you'll get real good at flying NDB holds.
Also, reserve time is set to drop dramatically here soon on the plane.
At 23, you have a decent shot of NOT getting the ATR. There are many 20-21-22 year olds in the classes. You will probably not get the CRJ anyways. But if you do get the ATR, the training dept. is high quality and is a lot of fun and the instructors are a great group of guys. ATR guys have all the fun. And 2 F/As, that means you and the captain don't have to share.
If you're under 2500TT, just remember that a couple of years ago, you'd be lucky to get an interview with Great Lakes on a 1900. BTW, you can make more money on the ATR than your CRJ200 counterparts can.