Anyone not on a Basic fare (point to point) can do a Same Day Change, so if someone is on a Plus fare (one with checked luggage allowance) then they can invoke the Same Day Change facility. Alternatively you can upgrade the Basic fare to Plus - probably won't be cheap - to get the same facility, otherwise pay what may be a £60 change fee and fare difference to go direct to the preferred service (but no further flexibility). Usually the difference between Basic and Plus is fairly small so it's worth bearing in mind if there is any chance of a change to plans. AA may (or may not) be amenable to changing their ticket, I can't see BA being willing to change the AA ticket unless there is force majeure.