Although it's never really an option given while your team is in season mode to choose an alternate costume, I've found that it's possible to do so. For example, if I were to choose the Lakers, during the controller choosing screen (where you can set which controllers to play on either the away or home side), set your cursor on the Lakers (assuming that you're using them). Hit R2 twice to get their retro jerseys (it works in both home and away games). Hit L2 twice to get their home white jerseys. This should work for all teams, assuming that you've unlocked the team's retro jerseys.

Refer to the code section of this game to find out how to unlock the codes.