For JAA, the conversion from (H) to (A) is exactly the same as (A) to (H) i.e. 10% of your hours can be credited up to max of 6 hours so the minimum time in which to complete is 39 hours. I believe you only have to do a Principles of Flight exam.
For FAA, it is an add-on to your licence.
Cheers
Whirls