Hello and welcome to JustAnswer.
I'm sorry to hear about your situation and hope I can help.
Unfortunately, the law pertains to all employers with employees in California, regardless of the number of employees the employer has.
Vacation days are considered to be 'wages' in California, and as such, it is illegal for an employer to eliminate wages that have already been earned, in this case vacation days that have become vested.
You would need to implement a separate policy in California to change it from a use it or lose it policy. However, you can place a cap on vacation hours in California. For instance, you could cap vacation hours at 80 hours and not allow employees to accrue additional hours of vacation until they use the 80 hours. This is different from allowing them to accrue more than that and then eliminating additional accrued hours.
I hope the above information is helpful.
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Thanks and best of luck!