Legal Question in Business Law in California

An employer in California can set, revise, increase, decrease, etc. employee compensation at will, except (a) not contrary to the terms of an express contract such as a union collective-bargaining agreement or an individual employment contract with the employee; or (b) retroactive reductions in pay are not permissible.

Without more details, I can't really tell whether what your employer did amounts to a retroactive reduction in your pay.

It sounds more like the employer was only giving you additional information regarding how the bonus amount is calculated each year end, rather than giving you retroactive (after-the-fact) notice of a reduction in earnings to which you were already entitled. This is in part because earnings cannot be known until the company's financial year ends, so expectations in, say, June, are just speculative.