As for American, this appears to be their policy:

If you are non-elite, you can upgrade with e-stickers at the gate (or placed on the waiting list) 4 hours before flight time and you can only upgrade on full fare tickets (I have experienced some exceptions).

If you are elite, you can upgrade 24/72/100 hours (gold, plat, exec plat) before flight time on ANY fare (including M class). 500 mile E-stickers are $35 for 7 or less, $25 for 8 or more.

In terms of upgrading ff tickets, I have never tried it. I usually purchase the lowest coach fare and upgrade with miles (currently 20k on American)