While fly4ever and Gator1999 have given you plenty of good info to think about, and I agree with both of them, I will offer my opinion:
While movement will take a while to happen, I believe that it is now only a matter of time until it does happen. Most likely the bulk of the movement will be into a 44-50 seat type. Since our FOs are for the most part not allowed to switch between types, FOs in a new type will be mostly new hires. The Captains, and where they come from will create the movement here. Unless we see scope relief from the NWA pilots, then there will not be a higher paying aircraft coming. With that in mind I doubt the senior Avro guys will take a downgrade in pay to fly a new type.
The way I see it, the first guys to go to a new type (44-50) will be mid level Saab Captains. With any new type the schedules will be poor until there are a decent number of planes on property. I see this as keeping the senior Saab folks waiting until the schedules improve with numbers, due to QOL issues.
The movement out of the saab will be somewhat offset by a reduction in saab numbers, but I think there will be vacancies. Those will be filled by senior FOs, and as of now I would have to say that the Avro has more of those folks, as many who were downgraded went to the Avro. There are plenty of senior saab FOs eager to upgrade as well and I think the sheer numbers of saab postions will mean you would have a better opportunity to get off reserve in the saab before the Avro. Just my opinion.
Either way, I think it will be a while before you see a line, so be prepared.
Another thing to think about is where you want to upgrade too, and which airplane. I have to say it is a big benefit to upgrade into a plane you flew as an FO, and at Mesaba there will be little chance to switch between airplanes as an FO (not impossible, but you have to be here for 3+ years without the chance to upgrade and then are subjected to a seat lock for 24 months). I have to assume that the Saab would be your first chance at a left seat position.
Either way, the plane will be great, both are a blast to fly although very different types of operations.
If you have gotten a class date, congratulations, and welcome aboard!!