The period within which may be commenced proceedings for the reduction of compensation on the ground of serious and willful misconduct of the employee, under provisions of Section 4551, is as follows:
Twelve months from the date of injury. However, this limitation shall not apply in any case where the employee has commenced proceedings for the increase of compensation on the ground of serious and willful misconduct of the employer.
Source:
Section 5407.5, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=LAB§ionNum=5407.5..