Can my employer change their sick leave policy and take away 5 years of accrued sick leave?

Generally, assuming that your employer's sick time policy is a bona fide sick time policy (meaning that it can be used only for illness) rather than PTO or vacation (which may be used for any reason), your employer may change its benefits policies and require you to forfeit your unused sick time. However, if the sick time policy permits you to use it for any reason (and not only for illness), then it will be treated like PTO/vacation. In CA, PTO/vacation time may not be forfeited once earned.