As a pianist, I would say the crossover from piano to keyboard is much easier, not the other way around. There is a certain level of dexterity and strengthening that comes with playing the real thing that you don't get with playing a keyboard- even a weighted one. There is J. something about a real piano that cannot be reproduced in a non-hammer and string instrument.
Also, probably the rock lessons are going to be much different than a traditional piano lesson. I would assume that rock keyboard theory is geared more towards learning chords, chord progressions, and using the piano as more of a back-up, supportive instrument. In a lot of rock music today, the piano is seen as more of a percussive type of instrument, providing beat and filler, not melody or lyric. I think that learning classical and chords are both important, but the meat of the instrument and theory comes with the more traditional training.
Anyway, I hope this helps!