1. Make sure the only save you have for 2K7 is a roster save (until after the first time you do the glitch).
2. Make your edits and save it.
3. Go to Xbox Live online desk online options/lineups.
4. Go back to the main menu and load your updated rosters.
5. Go back to Xbox Live online options/lineups and check your lineups

If it doesn't work the first time just restart the game and go straight to the online lineups and follow from step 3.