I think you will find that anyone who has switched from a UK/CAA CPL/ATPL to a JAA, they no longer have this privelage.
Are you sure about that? Not that I know for certain...although the impression I got from all the discussion we've had on here about EASA/IMC/etc etc was that yes, current JAA ATPLs could hold an IMC rating (In fact I'm pretty sure that's the arrangement my instructor had...ATPL+Multi crew IR+IMC rating for the single pilot stuff).
My PPL is JAA...with an IMC rating. Why should it be any different for ATPLs?
They do have to take the IMC rating test though...which is something the old CAA ATPL I think allowed you to bypass.