I had a OWE issued by AA with ARN-LHR on BA as the first segment in early August. The day before departure (and after ticketing) I called AA to change the date of departure and had no problems at all. When I got to the airport BA put a sticker in my ticket reflecting the change. I believe this was done by the rules, not an exemption. But, like R2 said it seems to differ depending on the "area" of origin.