You can probably get on the late flight to MIA (it leaves at 5:45) and then take a flight to STL the next morning. You would have to pay for a hotel in MIA though.
I think you are either stuck leaving early or stuck spending the night on the way back away from home somewhere.
AA is usually pretty good about accomodating schedule changes. Since you are on an award ticket though, it maybe tougher to change.
Ultimately, you can cancel the tickets and get your miles back for free. I am guessing this is not your preferred option though.