It can be tricky and a bit of a nightmare for everyone, usually because of the time constraints at the gate. If you really cant bear the thought of a C middle seat then ask the staff if there is anyway to be put back to your original seat (note I say ask, not rant like a crazy person and put everyone in a bad mood, not that I am saying you would, this is a guide). Normally the staff are happy to ask the person moved into your seat to swap. Often the last minute onloads are zero status and will be so excited to travel in club they will be happy to help. Alternatively we may look for any staff passengers with an aisle or window in M and move them fo you. I have dealt with this a few times and never had a problem other than time.

On a slight tangent I remember once upgrading a J pax to 4k. He went bonkers announcing that he knew what First was like, he had flown it on Continental, and he wanted his pre booked aisle back. I tried pointing out that firstly this certainly was not CO, secondly he had three windows to himself and quite a bit of aisle to boot. He remained unmoved by my sales pitch, so I switched him with a charming Silver card holder and off he went. A few moments later (the time it took him to board and glance left) I heard the sound of running feet and very out of breath (we were on the victors) he appeared beside me and told me that he had changed his mind and would accept F (how nice of him). I had to tell him (without looking to pleased) that sadly I had filled the seat we had offered him and he would be travelling in J. Contintental, I ask you.....