I have a similar question to the OP, except from experience I know that if I use the corporate booking system then I won't be able to do anything with the ticket on the AA site.

My question: Does anyone know if I book a cash fare in coach (W) on the AA site can I then upgrade it to first (P) after ticketing, and will it be for the same price difference compared to the quotes I get for each option?

For example if the trip is quoted as $300 in coach (W) and $500 in first (P), will the upgrade cost be $200? If I do the upgrade immediately after ticketing I would assume there wouldn't be any change fees as it'll be within 24 hours of booking?