I was thinking the same thought a while back, you might be able to replace Roland keys with Casio keys, but you would not be able to replace Yamaha keys with Casio/Roland/Korg/Fatar because they wouldn't fit in the keyboard. Yamaha keys are slightly smaller over all.
Starkeeper

P.S. Is it my imagination, the key feel on the new Casio WK3500 was exceptionaly good, even better then my Roland EM keys.