BAD EXPERIENCE

I REALLY HAVE A BAD EXPERIENCE WITH UPGRADING ONLINE WITH MTN. AT FIRST, I HAVE A CONTRACT WITH MTN SINCE 1997. NOW THAT I AM UPGRADING, MY ID NUMBER DOES NOT CORRESPOND. MY QUESTION IS, HOW DO I HAVE A CONTRACT FOR THIS LONG AND NOW SUDDENLY MY ID NUMBER IS WRONG? VERY VERY DISSAPOINTING!!