This is a simple claim. It is required under EU rules and the terms are spelled out in the Regulation. Downgrades are the easiest. There are no exceptions to the provision as there are with delay compensation and the calculation is simple. This makes the claim short & sweet and AA's EU team is well-trained and (relatively) efficient.
Things slow down and become more contentious on certain complicated claims, but downgrade reimbursement (refund) is not one of them.