If you are signed up to a GA ladder team you must make sure that your GA ladder nick matches you in game nick name. I'm pretty sure you can just change your nick but not your login name.
"Too many disputes arise as a result of players changing in-game names without updating their name on the GameArena website. This causes unnecessary, time consuming disputes that do not impress either team or the GameOps and as a result, any player who does not keep their player name up to date, resulting in an unnecessary dispute, will cause their team to forfeit the match" - GA ladder rules