The problem happens because when you do a MFU, your storing the fare in one cabin but booking someone in the next cabin above. As long as your have a seat assignment in the cabin that you should be in, then your all set. The system sees that your booked in a cabin at a fare that does not exist in that cabin. Conflicting info.
[This message has been edited by BRITISH AIRWAYS USA (edited 09-15-2003).]