As a GLD or PLT you'll have to accumulate enough 500 mile stickers in order to upgrade. For example, after you buy a regular cheap economy ticket for a 1600 mile flight you'll still need 4 stickers to get a chance to be upgraded. AA will issue you 4 stickers after you fly every 10,000 miles as an elite. You can also buy stickers for $30 each. As you can see even if you're lucky and there are available F seats on the flight, you won't be able to upgrade for free on most of them.

Most people posting here are EXP, to them these considerations do not apply. After my next flight I'll have 12 stickers in my account, received by flying only, no promotions. Still haven't had a chance to be upgraded with them.