Not the issue here though, the op wants to replace his chassis with the one from vehicle b - if he keeps vehicle a's identity and scraps vehicle b's identity he has...................... simply replaced his chassis. Exactly the same as if he had bought a new replacement chassis - no? Seems this "can't use a 2nd hand chassis" is very sweet for the new chassis manufacturer.
Doh! That's exactly what you can't do (legally). Everything should be taken off a and put onto b then a should be scrapped or rechassised.
It seems quite simple to me but here explains it better than I can...
http://www.glencoyne.co.uk/vosalaw.htm