Got it. I've never gotten far enough into a season to know that... maybe it's from my old Fantasy Draft days playing Madden, but I my attention span doesn't last longer than a couple of seasons, haha. That in mind, though, if you're going to just start a new dynasty to play with this roster -- like I said, I'd probably just recommend you make a new install with the MVP '12 TC as long as you have the hard drive real estate to spare. Yankee4Life has a really easy guide on how to do this here: http://www.mvpmods.com/index.php?showtopic=43843.
If you're dead-set on sticking with your MVP '08 dynasty without the need for an additional MVP installation, I'm not sure if that's possible (or even how to start to fix that). I do know that, when I've had old saves from previous rosters/TCs that have carried over to newer installations, the rosters have stayed the same (I believe the roster data is preserved in the Dynasty/Owner file). For example, before nani1379's awesome rehashing of Target Field, I used dennisjames' CAS version, which was just an owner mode save with the stadium fully upgraded. When I've loaded that Owner Mode save, the MVP '08 roster is preserved, even though I've used the CAS Target Field on the MVP '12 install. You *might* be able to just copy your Dynasty save from your MVP folder in My Documents to the MVP '12 directory, but I'm not sure how that would handle crashing/cyberfaces and whatnot.