The Democrat is well known after a run for the Senate last year. But former Vice President Kamala Harris' decision about whether to run looms over the field.
In order to change your political party preference, you must re-register to vote. You can re-register to vote by completing a voter registration application online at RegisterToVote.ca.gov.
In California? You can do it online.
https://www.sos.ca.gov/elections/frequently-asked-questions
https://registertovote.ca.gov/