I also think it is important to remember that YOU CAN use a pre-ban upper on a POST BAN lower just so long as you don't fall into a category where you are committing a crime according to the Crime bill!
If you were to remove the mag release button and replace it with a nut as had been discussed on the old forums they you NO LONGER have a removable magazine as tools would be required to release the magazine. Therefore, you have a POTENTIAL legitamate use for this upper which would NOT be illegal (((According to my Interpretation of the law)))!
Additionally!!! I have an 16-inch Preban upper that I use on my POST BAN Eagle Arms lower which I built without a gas tube! This upper is a T-Bolt action upper and is 100% legal and I even use it to hunt here in PA as I can't use a semi-auto here! I also can add my Ciener 22-conversion and put this upper on my M-16 and shoot full auto! Now, I CAN NOT use the ciener 22-conversion on the post ban lower as that would be a SAW as defined by the crime bill!
As long as you have a legitimate LEGAL use for a part..... the ATF or other folks are not going to have a foot to stand on in court! Now, if you are dumb enough to put a Pre-Ban upper on a post ban lower in a way that creates a SAW...... AND you get caught...... that is a whole different story!
Hell, I have an 11, 14.5, 16, and three 20-inch uppers. None of them are Post-Ban. I do have a Pre-Ban, Post-Ban, and a registered M-16 that allows me to have legitamate use for any and all of these parts!
Good Luck!