CA PC 12020(c)(25)(A)
It is 100% legal to import/manufacture/possess/sell/etc. a 10 round magazine that has been modified from a large capacity magazine. However, the large capacity magazine needs to be permanently modified to 10 rounds or less.
If you are just coming to visit CA, get 10 round magazines.
Also, if you are a non-resident of CA and you bring your large capacity magazines to CA, then you are guilty of importing large capacity magazines, which is a felony [PC 12020(a)(2)].
CA Penal Code 12020
(a) Any person in this state who does any of the following is punishable by imprisonment in a county jail not exceeding one year or in the state prison:
(2) Commencing January 1, 2000, 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.
(c)(25) As used in this section, "large-capacity magazine" means any ammunition feeding device with the capacity to accept more than 10 rounds, but shall not be construed to include any of the following:
(A) A feeding device that has been permanently altered so that it cannot accommodate more than 10 rounds.
(B) A .22 caliber tube ammunition feeding device.
(C) A tubular magazine that is contained in a lever-action firearm.