There’s the minor problem of Art 4 Sec 3 in the Constitution:
Section 3
1: New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
Fine. Northern half and central valley keep the name California. Shove the coastal group into the ocean as independents with a new name.