mad_jock thats correct IO
But it gets a bit funny after that for Commercial pilots.
You can't just go and do the IR only exams it needs to be the full lot
Well not quite, you do get credited VFR comms for having a CPL so just the 13 to sit!! Saves a whole heap of study that does....
MJ is correct however, if you just do the 7 IR exams and add an IR to your CPL you end up with a CPL/IR but not a Frozen ATPL which is where it gets interesting.....