The bottom line is that CA is an at-will employment state. They can change job title, salary, hours, duties as they please. If you do not like changes your remedy is to quit. Similarly, they can terminate you for any reason they want except a discriminatory one.
In short, you have no legal recourse against them. You can stay and negotiate the new terms with them or you can resign.
The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change. Attorney is licensed to practice law only in the State of California. Responses are based solely on California law unless stated otherwise.
If you've really given up other opportunities, and turned down jobs you were offered, then you might have a claim based on your detrimental reliance pm this employer, even though you were an "at will" employee.
Can you still pursue your other opportunities? If so, you should do so, and that would pretty much negate your damages. But if not, see an employment litigator to fully disclose the facts of your situation.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.