If you owned 13 round mags for the G21 20 years ago when you left, then you can legally have them in California. If you had the G21 it's likely still registered, which does have certain legal benefits should you have a bad encounter with the law.
I have never thought about this before, but is that an established precedent? If so its very interesting, but I have sincere doubts.
I did not know they would allow hi-cap magazines back into the state. Maybe, but I never heard of it. I also have doubts.
It happens to be the law:
http://www.leginfo.ca.gov/cgi-bin/displaycode?section=pen&group=32001-33000&file=32400-32450
32420. Section 32310 does not apply to the importation of a
large-capacity magazine by a person who lawfully possessed the
large-capacity magazine in the state prior to January 1, 2000,
lawfully took it out of the state, and is returning to the state with
the same large-capacity magazine.
32310. Except as provided in Article 2 (commencing with Section
32400) of this chapter and in Chapter 1 (commencing with Section
17700) of Division 2 of Title 2, commencing January 1, 2000, any
person in this state who manufactures or causes to be manufactured,
imports into the state, keeps for sale, or offers or exposes for
sale, or who gives, or lends, any large-capacity magazine is
punishable by imprisonment in a county jail not exceeding one year or
imprisonment pursuant to subdivision (h) of Section 1170.