That's bullshit. If you have REGISTERED "assault weapons", then you can do all the AWB no-no's to your gun. Collapsable stock, Flash Suppressors and the DREADED bayonet lugs...
What we CAN NOT do is buy new normal capacity magazines. Those are banned by a STATE level law.
Here's what people are probably thinking (though I know some people haven't thought about this one):
You also can not convert a gun that was not previously registered as an "assault weapon", such as a post-ban CA legal M1A, into a "no-ban" gun as that would violate the 2000 CA ban. For example, my buddy has a rifle stocked post-94 Federal AWB Benelli M3 Super 90 (semi-auto and pump shotgun...) He can NOT install a pistol grip stock on this shotgun because it was never registered as a CA "assault weapon" (why would he have registered it when it didn't "qualify"?) and if he did install the pistol grip stock, he would be breaking the 2000 CA ban. Now if he moved to Nevada and did it, it would be 100% ok.
Bottom line and just to repeat:
1) If you have registered "assault weapons", they can become "no-ban" guns no matter if they were pre or post 1994 build. I called the DOJ on this a few times to verify. Talked to three different agents.
2) If you have un-registered "assault weapons", they're illegal no matter what configuration you have them in...I HIGHLY suggest removing the pistol grip immediately.
3) If you have guns that aren't, in their current configuration, an "assault weapon" by CA law (the 2000 CA Ban), you can NOTconvert them into no-ban guns because that would violate the 2000 CA ban. If you registered your non-"assault weapon" gun (why someone would do this, I don't know but you never know!), then you COULD make it a no-ban.