I'm AA PLT, and am unaware of an "automatic" upgrade, unless you are EXP.

To get your upgrades several things need to take place

1. You need to call the AA GLD desk and request to be upgraded for each segment
2. They will put you on a list which will upgrade you 24 hours in advance of that flight (72 hours for PLT).
3. You must have the appropriate number of "stickers" in your upgrade account. Each sticker is good for 800 miles of upgrade, so if a flight is 810 miles, it will take two upgrade stickers. You get 4 of these stickers for free every 10,000 miles you fly. You can purchase more online, at the check-in desk, or over the phone.
4. They will notify you if you request to be notified when the upgrade goes through.

The way they determine availability is that EXP get first shot, PLT gets second, and then GLD after that. It goes by when you made your reservation as to the competition between each level for upgrade availability, or so I am told.

5. The only other upgrade, is one where they need your coach seat to give on an overbooked flight, whent hey will give you an operational upgrade at no cost to you. I have actually seen this happent o a colleague traveling from ELP-ORD, after I had already used my upgrade stickers to get a FC seat.

Hope this helps, and it is explained at aa.com under the status levels, I believe.

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