You will of course have to go through immigration, collect your bags, re-check them, and take the shuttle train to the AA terminal. Then you'd need to check in (unless you can get a boarding pass on the separate PNR from BA), get through security, and board your flight to LAX.

Biggest uncertainties are the arrival time of the BA flight and the number of flights that will have arrived in the 20-30 mins before yours. I'd estimate that you'd make the earlier flight 25-35% of the time.