I think the question may be whether there are offers once you
complete the check-in process, in which case, I don't think I've ever seen one. The offer
during the OLCI process is your last chance.
That being said, before check-in, even if you don't see a current offer, you can always call in to up-fare and they should waive the change fee (a la GG BUYUP that you are used to
). I suspect they could do that after check-in too although they might have to undo the check-in first.