California Code, Elections Code - ELEC § 2221

(1) The voter registration status of a voter whose residency confirmation postcard is returned by the post office as undeliverable and who has no forwarding address shall be updated by the county elections official to inactive pursuant to paragraph (2) of subdivision (a) of Section 2226, and the voter shall be mailed a confirmation notice, as described in subdivision (c) of Section 2225.

(2) The voter registration record of a voter for whom a forwarding address within the county or outside the county is received shall be immediately updated by the county elections official to reflect the new address provided by the post office, the former address shall be maintained with the voter registration record, and the voter shall be mailed a confirmation notice, as described in subdivision (b) of Section 2225.