I just checked California site on voter registration.

If you switch your party affiliation YOU DO have to re-register to vote, which has to be done at least 15 days before an election.

So, if your party affiliation was changed and you thought you were just signing a petition against child molestation, as happened in the California case with the Republican scam, you would show up at the polls to vote on election day and find that you could not vote because you did not re-register.

http://www.sos.ca.gov/elections/elections_vr.htm