Originally Posted by Andy

My take on that exception is that it allows an FO to bid down to equipment that s/he wants to be CA on so that s/he can get some experience on the aircraft before bidding CA.

For instance, suppose you've been a 777 FO for 7+ years and want to bid 320 CA, but want to be familiar with the equipment before going through CA school so you bid 320 FO and fly the line for 6 months before bidding 320 CA. That's why I think they allow FO to CA upgrades on the same equipment even if you have a seat lock.