http://www.srg.caa.co.uk/documents/GID36.pdf
as i understand this document you will be able to retain the ATPL-theory if you are swithing from an national to a JAA-licence
(read note nr 4) but since english isn't my primary language maybe someone else has a point.
Why not transfer the ATPL theory over to the new licence right away ?
Why wait for the 500 hours multi-pilot aeroplane?