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.
Source:
Section 5700.609, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=FAM§ionNum=5700.609..