I called about something like this the other day. I'm flying back from Hawaii in November, and I will become EXP on my flight over to Hawaii. Obviously, I want my EXP status to be the one that counts. The agent I spoke with told me that as soon as it changes in my account, I get the new status on the upgrade que (though, let's be honest, it couldn't hurt to call AA and remind them politely that you are now a PLAT, EXP, etc).

Also, the agent told me that even after your status changes, you should be able to apply the original "time-stamp" for your upgrade request, meaning I might have priority over some other EXPs on that flight back based on when I bought the ticket and requested the upgrade as a PLAT.