I think that your PPL(A) exams should be valid for PPL(H) with the exception maybe of Principles of Flight (being a slightly different principle!!) and definitely the Aircraft Specific (since you will be a getting a rating in a different machine!).
Depending on time limits, your ATPL(A) exams should also be valid but you may have to sit the Helicopter Principles of Flight. There is very little difference between the exams for CPL(A) / CPL(H) and ATPL(A) and ATPL(H).
As for the actual flying bit - your PPL(H) minimum hours are reduced by 6 to 39. But beware, you may not do it in minimum hours; very few do!
Cheers
Whirls