I'm not one hundred percent on this, because it was a few years ago that I made that very switch -- Tyros 3 --> Genos. But I am pretty sure that the Genos changed the voices to something 'equivalent', and not the legacy voices. In many cases, it wasn't a good choice.
I seem to recall having to look up what the style originally had, and then replace it with a Genos voice. In my opinion, the Legacy voices sound about the same between the Genos and the T3, but the Genos voices sound much better. So whenever possible, I would switch to a Genos Trumpet, Strings, or whatever. To my ear, the drums were always better with a Genos kit.
Oh! Btw, I don't know if your styles used 'MEGA' voices, but if they did, make sure you find a Mega voice of the same family to replace them -- Guitar with Guitar, etc. The extra notes that trigger effects will sound pretty weird if used with a non-mega voice voice.
Hope that helps!
Sincerely,
Adam