First, I'd go check out what people are saying at calguns
are saying. They'll have far more reliable information.
Second, I'd guess that whatever the gunshop guy was saying was only partial truth. The state of California can't just change the definitions in the NFA, and have you register your AR/get a tax stamp for it. I also really doubt given the way the current law is written about magazines, that they would rewrite it to allow new standard capacity mags. As it is currently written the only magazines that are grandfathered must have been possessed by you in the state of California before the original ban.
I'd guess the most likely truth is that they may reopen the state AW registry for items currently possessed that they now consider offensive.