Ouch. Sounds as though I am totally screwed. Do you think I could book transpacific in economy with my AONE4 and upgrade in cash to J?
It is not offered or permitted in the rules. It could be offered by LAN itself at the airport, however, that I don't know. I doubt, however, that LAN will consider L as a valid upgrade platform (I realize you paid A, but you'd be flying L and that is what LAN will get for that segment).