At the main menu, hit the following sequence:
Left + Circle, Left + Triangle, Left + Square, Left + X
Note: Works for a Created player only
Enter the following code at the Main Menu:
Right + Square, Right + Triangle, Right + Circle, Right + X
You should now have this new boxer in your line up.
Name your boxer PSIRULES, then in Career Mode fight one match. Then leave and come back and your boxer should be upgraded.