Uh...I thought that if you can have OPEN segments on a TKT (which is rare these days), it always says something like CXOPEN or AAOPEN on the TKT. The carrier code is needed exactly for the purpose of calculating YR / YQ. Otherwise as OP said YR / YQ cannot be calculated. Also you need carrier codes in the Fare Construction field. One might then be able to change an AAOPEN segment to a BA FLT if fare permits so, and I guess whether extra tax needs to be collected then depends on who re-issue the TKT.