FIFA 4 Fans has a nice download for the FIFA 12 Demo. Their modder Sandro has created the Arena Changer for the FIFA 12 Demo which allows changing the players in the Arena Mode. The tool is a exe file which you have to run. Then you can select the goal keeper and the field player depending on which players you want to have in the arena.
Just run the exe-file, choose the players (keeper and field player) and save.