Originally Posted by MADPhil

Sorry I wasn't clearer but that is what I meant. The retroactive fix can be done but is much more hassle. In the case I mentioned I finally got it fixed in the FL at ORD after flying MSN-ORD. The agent at check-in tried to change from AA to BA but didn't reprint the BP. I checked online at the gate, found my BA number wasn't showing and the agent there tried to change it but also didn't reprint the BP. The FL agent fixed it easily, including the first segment since it hadn't posted yet. This is why it's good to have the correct FFN number printed on a BP just in case!