Most of the migration from the NorthEast to California took place in the 50s though 70s, and whil it did have an impact, that impact was largely done by the late 80s, and was limited to a relatively few geographic areas such as West LA, Santa Barbara, San Francisco, Marin and parts of San Mateo counties, and again, even as recently as 1990 with the exception of San Francisco and Marin, there still was around a 45% Conservative base in these areas.
What one has to do is compare the 1988 election map to the 04 election map, and see what drove the change, and its simple, white working class voters in the 90s largely left California, and in their place were mostly voters who were immigrants, and this shifted California to the left. Even in my neck of the woods, the Bay Area, had fairly large pockets of Republicanism, even conservative Republicanism up till 1990, but as formerly conservative bastions such as South San Jose, Sunnyvale for example, changed demographically, their politics shifted to the left. The impact can be seen in an even more dramatic fashion in Southren California.