Can my employer simply notify me one day that he has changed my salary?
If your employment contract is "at-will" California courts have held that employers may change the compensation level, because it is tantamount to ending one agreement and starting a new one. If you have a contract that guarantees a certain income for a period of time, then this probably can't happen. If your the member of a union, you should certainly talk to your rep to see what's up.
However, it should be noted that if you feel this has happened for an illegal reason (race, gender, etc.) it is also a no-no even if you are an at-will employee.
If you're an employer, you should review the circumstances carefully to make sure that your actions aren't having an illegal discriminatory effect or somehow violate a contract.