Under the JAA PPL, if you want to convert from fixed wing to rotary, then you have to do the majority of the rotary PPL. I think you get something like a 5 hour dispensation, but that's it. Of course, you won't have to do the RT exam again.
From a commercial perspective, people have tended to move from rotary to fixed wing, since the requirement, at ATPL level, is straightforward, and not unduly expensive. I suspect fixed to rotary is more difficult (and expensive). Rotary to fixed moves are normally done for more money, rather than the joy of flying. Most rotary people would prefer the flying skills required to hand-fly, rather than be concerned that your coffee has the correct amount of sugar: the rotary man's view of an airliner pilot.