The latest proposals for the transition are in draft form at the moment, I imagine they will be published soon.
The CAA are currently operating an 'interim procedure' because they don't have proper JAA helicopter exams yet. This involves sitting fixed wing ATPL papers in all subjects except P of F and Performance and a special CAA/JAA rotary P of F paper.
They now expect the new style exams to be available from 2003 and will require new courses sold after a particular point, probably towards the end of the year, to comply with a syllabus that contains more helicopter stuff in subjects yet to be specified. Those that start a course before the changeover point will be allowed plenty of time to finish on the interim procedure.
The dates for the introduction of the helicopter exams have already been moved forward several times so they may be moved again.