So what happens to preexisting old large mags?
Turn them in, destroy, send or sale out of state. Current legal hi-cap pre-existing mags cannot be sold or transferred in state.
Banned and subject to confiscation.
Pray for the Free Californians.
Maybe they escape and prosper elsewhere.
CA is done.
Same thing as after the first "assualt weapon" registration laws decades ago: most are stashed away.
Some day I will try and find out just what % of owners actually did "register" back then. I sent mine out of state for safe keeping.
I do remember hearing a police call on the scanner some time after that period, and the dispatcher telling the responding cops that there was an assault weapon registered to that address. The cops were using the registration info, regardless of the bleating of politicians.