California Code, Penal Code - PEN § 969a

Whenever it shall be discovered that a pending indictment or information does not charge all prior felonies of which the defendant has been convicted either in this State or elsewhere, said indictment or information may be forthwith amended to charge such prior conviction or convictions, and if such amendment is made it shall be made upon order of the court, and no action of the grand jury (in the case of an indictment) shall be necessary.  Defendant shall promptly be rearraigned on such information or indictment as amended and be required to plead thereto.