Do you mean changing from a Transport canada CPL to a JAA CPL? I think that you would need to complete all 14 ATPL exams, or I think for just the CPL it's about 9 exams but that would limit your options regarding employment. You would also need to do flight training for the CPL skills test and the IR, I think that it's just hours required at the discretion of the head of training for the CPL and a minimum of about 15 for the IR. I'm sure somebody with more knowledge on this will correct me here.
Check out 'LASORS' on the CAA website, it's an online reference for those sort of questions.
Also have a look at the wanabee section on here to get an idea on different schools and costs involved.
Time consuming and expensive is all I can say with any certainty!