I just received the following response from the CA Attorney General regarding this issue:
Your request for information has been received and reviewed. Indeed you may change the non-serialized parts that are attached to your lawfully registered assault weapon without violating the law, so long as the parts you are adding are not themselves alone to be considered prohibited devices. (i.e. silencers, grenade launchers) If you have any further questions or concerns please feel free to contact the Firearms Division at (916) 263-4887.
