Another reason why it is important (when you can) to book two one ways. That way the whole trip isn't affected when this happens.

This post was good reminder for me as I always used to book one ways when upgrading. I had the split happen to me in June outbound YYC-YOW-YYC on a two-way ticket on a family booking but fortunately had booked business class for the return portion. In the end, all three of us got upgraded on the outbound but the split in the reservation had already happened. Caused some issues at the MLL (adult daughter) but we worked that out as well.