No problems at all switching seats. I did it to get my Command Endo even though I started flying as an FO. Just jumped in the LH seat for the rejected take-offs.
Its easy to do if you're on a course with two FOs. As one will always be playing captain. It was during this time in the left I was able to experience enough rejects to satisfy CASA requirement.
On the subject of Co-Pilot Ratings. Yes, many countries out there don't recognise them. If you plan to work overseas, it's always best to get the Command Endo as they makes conversion easier.