I just wanted to clarify and flesh out the potential DA argument, to make it more persuasive.
By replacing the upper receiver, you have transformed your rifle into one that no longer exhibits “minor differences” compared to the “Colt AR-15” listed in the AWCA and the AG’s list. Therefore, you have created a new rifle not covered by your existing registration. Because it is a new rifle that contains a detachable magazine and a banned feature, i.e. flash suppressor (among others), it is prohibited by the AWCA.
The legislature clearly intended to control and to restrict the proliferation of assault weapons in California. Although the list mechanism was a political solution, the lawmakers attempted to mitigate the weaknesses of that approach by inserting a provision for the AG to add additional named assault weapons to the list. In addition, the legislature amended the statute in 1999 to define assault weapons by characteristic. By installing a new upper receiver with a feature expressly banned, thereby creating a new rifle, you have introduced a new "assault weapon" into California, a result clearly at odds with the intent of the legislature.
Although this argument is a bit more persuasive, I still think (in a law student kind of way
) that your rifle is legal in light of the current judicial interpretation of the AWCA. Until the courts define what constitutes “minor differences,” and the AG comes up with a new name to distinguish your rifle from the “Colt AR-15 series,” your rifle complies with the law. Plus, I still can’t get over the fact that your lower is already legally registered.