I can tell you that I did recently change the date of the first flight of a AOWE, on BA, on a ticket made by a travel agent. On the one hand, the person who arranged the change was a friend of mine-- but OTOH, he said that his BA supervisor said the change was allowable-- despite the impression I was under that it was not. So it's difficult to ascertain with certainty if I was done a favor or not. In any case I think it is BA you will have to get to make this change-- not another OW carrier.