Not so simple. Bigger size (6x6) also means bigger lenses. I doubt it if lenses for 645 systems would work for 6x6.
Sorry, you misunderstood - of course 6x6 requires other lenses. But if you only turn a 645-sensor, that's no problem for the lens as the lens always produces a round picture. So turning a 645-sensor should be easy - but a square 56x56mm² sensor would not work. But I don't see such a sensor at the horizon (mainly because there is simply no camera around to use it, expect Hassy 500 and Hy6...).
Andreas