Did the same thing. 15-20 minute wait but managed to move three of us. The online system was a complete fail for me - just a series of error messages even when I tried to relocate myself only. My advice is bear with the queue time and just phone up. The girl that served me was also really helpful - I had the stadium plan in front of me and she advised on all the available options. ‘Got there in the end…I managed to phone in and get mine sorted. There was a glitch with the online system which wouldn't let me move two of the tickets.