Heliporto,
I did the ATPL(H) to ATPL(A) conversion under the rather dodgy interim procedures including the 'bridging exam' of 100 questions that the CAA raked together for me!
PofF Fixed wing, Operational procedures and performance were all done along with the said bridging exam.
As far as I am aware, and Alex would be far better placed to confirm this, if you have passed all of the ATPL(H) exams and are in possession of a CPL(H) then you can convert whenever you wish. Unfortunately it all looks as if it's up in the air again as the CAA blindly struggle with the implementation of yet another complex layer of licensing issues *sheesh*.
Good job the tests and licence issuing are cheap, hang on? Oh no they're not are they.
Good luck in the conversion!