Really appreciate the response normanataa. If compatibility indeed proves to be such a no-brainer, it would appear most JY/dismantlers are losing potential sales. The typical ebay and other ad listings usually specify something like:

In response to a question asked thru ebay, I even recall one (JY) seller flat-out stating the 722.350 would NOT fit earlier model 420/560s.

Naturally this made me cautious, especially when combined with the fact I'd not read of anyone on the boards attempting it. Yet I'd never read anybody categorically stating it couldn't be done, either. Hard to believe these would be uncharted waters