I won't repeat what's already been said. Needless to say, going from AA EXP to UA Plat will be a downgrade. I'd be surprised if you hit 50% upgrade rate. As a Plat, I struggled to get upgrades thru DEN. There are A LOT of 1k's there.

Another change for you is when you fly international, you need to buy a W fare at a minimum to even get a shot at upgrading and play the lottery. On AA, you can buy any fare and apply an SWU.