I believe you may get 5 hours flying credit off the 45 hour minimum requirement although most pilots need more time than that anyway to get to the standard.
The main thing to be aware of is that if you are flying both fixed and rotory wing and fixed wing is your main flying then you do expose yourself to some not insignificant risks if/when thing start going wrong in a heli.
In a helicopter when you have an engine failure your in built natural reaction has to be to pull back on the cyclic to flair the helicopter and lower the collective. The problem is that the stall reaction of an airline pilot is to instinctively push forward on the stick which has catastrophic and fatal consequences in a helicopter. It can be trained into you of course and there are many that do fly both but there are still a number of accident reports from low hours helicopter pilots with high fixed wing experience getting into trouble (check out this months Feb 2013 AAIB report that has another fatal accident of this type).
I used to fly both but decided to move entirely onto rotary to ensure my reactions in an emergency were ingrained - plus I prefer choppers
In terms of the exams I've no idea if they have to be redone in their entirety. You would have to do the Rotary theory paper and the helicopter type paper for sure.