Depending on how it is getting booked and if you company would allow it and it wouldn't be a cultural upset might be possible to just pay the difference out of pocket?

My last company were happy with a screen shot of the lower fare, receipt for the higher fare and claiming the lower of the two

New company uses a corp TA but no problem to get a quote for Y and J and have the difference routed to your CC (or if we want to extend trip for personal reason and there is a fare difference etc) - but we are clearly told that the same will apply for a change fee - which could get very painful if it was a high Y fare class and you booked a non-changeable I!

Personally I wouldn't be happy to do it until I was stood at the desk and could AUP, but even then considering I recently offloaded myself from the seat I was already sat in as plans changed last minute that'd be a risk, but depends on your situation