What people forget is that to convert later from CPL to ATPL you've got to take all 14 exams AGAIN!! (with the possible exception of the comms exams).
Do you mean converting a fATPL to a full one requires the retaking of all of the exams? (As well as the 1500 hrs requirement etc.)
I think the difference between CPL and ATPL is technically the weight of the aircraft you are flying (off the top of my head 5,700kg but I may be wrong). I have heard of people flying multi pilot ops in what would normally be single pilot aeroplanes, but I doubt that this is officially multi-crew.