Can an employer in California force me to use vacation time so that my accrual can comply with a new PTO cap?

My employer has changed the cap on PTO as of this year and is asking me to take vacation time because I have far more hours accrued than the new cap allows. Is this legal? Is there such a thing as a "Grandfather" clause in this instance? Is it mandatory (legally speaking) for me to use vacation time to comply with the new policy?