All of the prior styles will play just fine on the T3. There will be some T3 styles, and obviously, voices that will not play on the older keyboards.
Registrations should not be a problem, and I suspect that Michael Bedesem's latest version of Regfix will likely be addressing the T3 when it becomes available in the states.
Same holds true with midi files, which Michael's MIDI Player program does an outstanding job of batch converting.
There may be a minor tweek that would be applicable to some styles, but overall, the work will be minimal at best. It's not as if you are changing brands or making major changes in operating systems. Kinda' like upgrading your computer from Windows XP Home to XP Pro--not a big deal. For techie types such as myself, I enjoy the challenge and consider this type of task as fun--not work.
PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!