If (1) the employer has a written set of rules under which promotions are applied for, considered and granted, (2) you had knowledge of the rules before you were promoted, (3) and the employer has violated those rules, then you would have recourse via a breach of contract claim against the employer. Otherwise, Cal. Labor Code 2922 controls and the employer can effectively do whatever it wants, which includes changing its decision, "at will": at any time, for any reason, or for no reason at all.
Note: if this is a government or union job, then you may have other rights based upon your union contract or through a grievance process concerning classified employment advancement (civil service). For any other employer, my answer is valid.
I realize that this probably isn't what you want to read, but the best I can do here is to save you time in looking for a legal resolution, where none may exist. Review your employee handbook and any other written policy guides concerningn promotions and see if you can find something that the employer may have done which violates those rules. If you find something, then you may have legal recourse.
Please let me know if I can be of further assistance.
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. Thanks and best wishes!