I have firearms bought prior to the current 10 round cap ban. I can keep them, unless I live in LA. Can I drive thru LA city limits with my mag that is legal in the rest of the state?
No.
That's one of the 'neat features' of the ordinance. (Also illegal in San Francisco and Sunnyvale - please be careful and circumspect.)
For the non-CA residents, here's the deal with magazines in CA:
1 - in 1999, the state passed SB 23, which gave us both 'by feature assault weapons' and the large-capacity magazine ban. A 'large-capacity magazine' is defined in Penal Code as one that holds 11 or more rounds. (Folks are free to argue that 'using the terms of the enemy' is bad politics, and I'll even agree with you - but in a legal discussion, using anything but the terms defined in law is inaccurate and confusing.)
2 - Possession, ownership and use were not (and still are not) criminal acts, for those magazines possessed in-state in 1999 and earlier. Transfer from a person was defined as a criminal act, as well as manufacture and importing. Recently the PC was amended to include 'receiving' a large-capacity magazine as another criminal act.
LEO are exempt, and there were a few other exemptions; a LEO who obtained large-capacity mags while employed, and then retired or changed to a non-sworn/non-LEO could keep the mags.
3 - For a long while, repairing those pre-2000 magazines was legal, and one could purchase parts kits. But in 2013, the legislature passed AB 48; effective in 2014, it became illegal to import or sell or buy a 'large capacity magazine conversion kit'. No one quite knows what those are, and there is not yet any case law or even, AFAIK, a solid legal opinion on the point. Again, repair kits and other collections of magazine parts possessed in 2013 and earlier were unaffected. Assembling such parts kits is thought to be 'manufacturing'.
4 - In November, 2013, San Francisco thought it would be fun to ban large-capacity magazines, effective April 7, 2014. See the
NRA article; lawsuit,
SFVPOA v San Francisco.
The city of Sunnyvale also banned large-capacity magazines: lawsuit,
Fyock v Sunnyvale
5 - Now, seeing the courts foolishly allowing other cities their bad law, Los Angeles has joined the fun. Just for completeness, here's the ordinance text:
http://clkrep.lacity.org/onlinedocs/2013/13-0068_rpt_atty_06-26-14.pdf (That one may be slightly amended, that's the proposed, pre-vote version. Be sure to skip directly to page 7 in that PDF.)
6 - State pre-emption is very narrow, and simply will not help here; see the wiki article --
http://wiki.calgunsfoundation.org/State_Preemption_of_Some_Gun_Regulation