It's all speculation at this point but I think there's a good possibility you're right, considering how 2K handled last year's DLC. Then again, to take an optimistic view, NBA Live also used to allow legends to be pitted against currents in their dunk contests.
Speaking of Live, one of the really small aspects I liked about their dunk contest: the ability to choose any of my players's current, retro, or practice jerseys. I'm hopeful 2K13 doesn't force players to wear specific jerseys.