Cal. Gov. Code § 23376
The officers of each affected county shall immediately execute and deliver to the board of supervisors of the proposed county copies of all assessments or other proceedings relative to the assessment and collection of the current state and county taxes on property in the proposed county. The copies shall be filed with the respective officers of the proposed county who would have their custody if the proceedings had been originally had in the proposed county and shall be deemed originals. All proceedings recited in such copies shall be deemed original proceedings in the proposed county, and have the same effect as if the proceedings had been had at the proper time and in the proper manner by the respective officials of the proposed county. The officials of the proposed county shall proceed with the assessment and collection of the taxes as if the proceedings originally had in the affected county or counties had been originally had in the proposed county.
Ca. Gov. Code § 23376