Can't seem to figure this out - any help appreciated. I have 100,000 miles on both programs. Trying to arrange special trip with granddaughter from St. Louis, Missouri to London. Can plan one year out, dates very flexible. I would like to buy full-fare economy and use miles to upgrade to Business Class, but how? BA fees are crazy, website is worse. AA won't allow an upgrade until after ticket is purchased, and then only if seats are available. Bit of a risk there! Perhaps AA to Newark or NYC, then use BA miles on Aer Lingus to Ireland (avoid BA fees) and somehow get from Ireland to London? Long problem - thanks anybody.