California Code, Penal Code - PEN § 30920

(a) Any person who lawfully possessed a firearm subsequently declared to be an assault weapon pursuant to former Section 12276.5, as it reads in Section 3 of Chapter 19 of the Statutes of 1989, Section 1 of Chapter 874 of the Statutes of 1990, or Section 3 of Chapter 954 of the Statutes of 1991, or subsequently defined as an assault weapon pursuant to former Section 12276.1, as that section read at any time from when it was enacted by Section 7 of Chapter 129 of the Statutes of 1999 to when it was repealed by the Deadly Weapons Recodification Act of 2010, shall, within 90 days, do one or more of the following:

(b) Notwithstanding subdivision (a), a person who lawfully possessed a firearm that was subsequently declared to be an assault weapon pursuant to former Section 12276.5 may alternatively register the firearm within 90 days of the declaration issued pursuant to subdivision (f) of former Section 12276.5, as it reads in Section 3 of Chapter 19 of the Statutes of 1989, Section 1 of Chapter 874 of the Statutes of 1990, or Section 3 of Chapter 954 of the Statutes of 1991.