It can only be done on the default rosters. I play with Oregon, and Walter Thurmond and Derrick Jones are both #6, and Chris Harper and Marvin Johnson are both #1. In real life, Chris Harper is #3, and Derrick Thomas is #1, so when I went to swap their numbers, it wouldn't let me since there was already a #1. Here's the funny thing, though. Since EA's raters can't seem to get player ratings right, and they made Harper a slow RB instead of a fast QB, I switched him to DE in the off-season, where he went from a 76 OVR running back to an 86 OVR DE. But I left him with #1 so as to avoid any kind of roster glitch, and now I have two #1's starting on my defense.