Family Code § 5700.609
ARTICLE 3. Registration and Modification of Child-Support Order of Another State
A party or support enforcement agency seeking to modify, or to modify and enforce, a child-support order issued in another state shall register that order in this state in the same manner provided in Sections 5700.601 through 5700.608 if the order has not been registered. A petition for modification may be filed at the same time as a request for registration, or later. The pleading must specify the grounds for modification.