The rules are the same. There are a few obstacles, though.
1. I think that your TA has to call, and it sounds like your TA can't be bothered.
2. It must be the absolute identical category. Not just "D", for example, but 2D or 3D or 4D.
3. Most significant: Not in all cases, but in most, assignments are not made until the last minute, and by then there are no choices left.