Welcome to FT!

1st recommendation: While a 35 minute connection is doable, it can also be stressful for someone who doesn't fly a great deal. You don't say why AA changed your flight-may I assume it was a schedule change, either STL-ORD or ORD-LHR? If so, you can call AA reservations and ask them to change you due to schedule change .....

2nd recommendation:... to an earlier flight out of STL or a later flight out of ORD.

3rd recommendation: If the agent says no, or is unresponsive, say thank you very much,hang up, wait 2-3 minutes and call back and ask again-it will be somebody else. It may take several attempts until you get something you are comfortable with.

4th recommendation: Before you call, have a rough idea of what you want/what makes you comfortable (flying out of MIA, DFW, or JFK to LHR, an earlier flight to ORD, a later flight to LHR, etc.).

Good luck and come back. You will find that this forum can give you access to centuries of experience and a willingness to share (mostly) that experience.