One thing that may help the OP - and anyone for that matter - is that if the CO website shows you as upgraded, you should appear on the FAs final report. While the F cabin was full, as the OP stated, his name should still appear on the final report along with his seat. The problem can easily be resolved from there, as the FA would have seen his name on the F list, and he would simply show her his BP or ID, identifying him as the true occupant of that seat. From there, the FA can ask the person occupying the F seat for their boarding pass, and of course if it wasn't an F boarding pass, they'd be directed to any seat in the back (or, depending on the mood of the FA/GA, interrogated as to why they sat in F and potentially removed from the flight if they were simply trying to steal the seat). What may have happened is the GA in IAH directed the person to take the first open seat he saw, and he unfairly took an F seat, or the GA ignored the upgrade that was processed and put him in that seat. Either way, the problem could have been rectified. I think it's a joint responsibility between FAs and GAs to make sure who sits up front is who is supposed to be up there. Granted, there's only a limited amount we FAs can do, but we can still do a little such as checking the upgrade standby list when seats are available, and if the Final shows F as full but a seat is still empty, checking with the GA to find out what the problem is. Even in this situation, if I was the FA working the flight and a person came up claiming they were supposed to be in F and F was full, I would still ask for their name and check my paperwork. I'd agree that the OP's experience is definitely unfair and unacceptable, but we can't point fingers and be accusatory. We can only allow him to state the facts to WeCare, and hope that they will follow through and determine what happened. Regardless, I wish you the best in resolving your situation. Your complaint should standout and have more validity due to the screenshots which accompanied your message.