"changing it", not quite as easy as that. i think having an FAA IR gives you some hour credit towards the JAA IR, but thats all, you still have to do the training (on a twin turbine, or the B206 at norwich?) and pass the skills test.
and.. you need a JAA CPL, and... the 14 ATPL (or 9 CPL) exam passes before that.
in other words. it aint easy. see previous threads on this subject. i am just summarising.