this has happened to me and family many times, however the domestic portion was only three hours tops and the intl up'd segment was 12.

you will be on the top of the upgrade list and will be upgraded before any complimentary elite upgrades are processed. the bad thing about that is that LAX-EWR rarely has upgraded elites as the F cabin more than often sells out.

the other bad thing is that mileage standby is "officially" NOT allowed at the airport. that means that battlefield elite upgrades WILL go to elites before you unless you are also elite. In that case it follows the EUA hierarchy.

there is a catch. I have done this before. explain to the agent that you are waitlisted for FC and that your miles have been deducted. tell them you should be listed at the top of the list as a P1. some agents know this and will do it and others will not. it all depends on who you get.