I don't understand your question, but I think I know what it is... Are you asking about changing your LHR/MXP segment to the AA codeshare simply to try and get a seat assignment? If that's the case, don't bother. I have an ORD-LHR-PRG with ORD-LHR on AA in F and LHR-PRG on BA in Biz but booked as the AA codeshare. I still cannot get a seat on LHR-PRG and will have to wait until the T-24 OLCI. So if I understand you correctly and you're just hoping for an advanced seat assignment on LHR-MXP, there's nothing you can do about that. Changing to the codeshare won't fix it.