Since 2004 there have been over one million CA residents who have changed political party – some more than once.

The following graphic shows the number of switches in the PDI database for eight different periods between statewide elections since 2004. (clicking on the picture should bring up a larger version)
It is noteworthy that party switching really heats up before the General Elections.  On average the period between Primaries and Generals sees 20,000 party switchers per month, while the months between election cycles average at 8,000.  These figures were nearly double for the months leading up to the 2008 Presidential election.

That single period from the 2006 General to the 2008 Primary saw a shift of 120,000 voters to the Democratic Party.   That was led by 70,000 non-Partisans along with 50,000 Republicans making the switch.
This mass of lines can be simplified to just the gains made by each party.  This more clearly shows how from the 2008 spike until the most recent 2012 election it was the Non-Partisan voters who gained the most switches, with a brief period between the 2010 Primary and General where the most switches were to Republican.
An analysis of these party switchers reinforces some common wisdom about the electorate and how it is changing.  The shift is predominantly to Democratic, although that is primarily from a brief 2008 bump.  The more consistent shift has been to Non-Partisan, with a lower number to Republicans.

The Latino strength among Democrats is known, but Asians perfectly match them in shifts, with 44% of movers shifting to Dem registration, 27% to Republican, and the remainder to Non-Partisan/Other.  The Democratic strength with the online re-registrants where Dems lost 9,878 and gained 17,483 from party changes.

Non-Partisan, including other small parties in the “Other” category has grown by the widest margin among the state’s Motor Voter registrants.

To Democrat To Republican To Other
Latino 44% 27% 29%
White 34% 30% 36%
Asian 44% 27% 29%
African American 41% 25% 34%
Homeowner 41% 25% 34%
Over 40 36% 29% 35%
Motor Voter 35% 23% 42%
LGBT / SUPPORTER 57% 10% 32%
Online Registration 46% 19% 35%
TOTAL 38% 28% 34%