Unfortunately, as an at-will (non-union) employee, your employer can demote you from a management to a non-management position at any time for any reason (as long as it's not a discriinatory one) with or without any prior notice.
This stems from the employment at-will doctrine, which is codified in California Labor Code Section 2922, and states:
"An employment, having no specified term, may be terminated at the will of either party on notice to the other. Employment for a specified term means an employment for a period greater than one month."
However, since you were basically termianted from your old position and offered a significantly lower paying and lower prestige position, you would be eligible for unemployment benefits for denying the demotion offered to you.
However, if you have any reason to suspect that discrimination against you due to a protected characteristic (race and age) played a role in your demotion, you should file a complaint
with the Departement of Fair Employment and Housing for discrimination in violation of the Fair Employment and Housing Act.
You can do so using the information available online here:http://www.dfeh.ca.gov/Complaints_eCompProc.htm
The Department of Fair Employment and Housing will investigate the complaint on your behalf,
and either file suit on your behalf or issue you a right to sue letter so you can pursue your claim
against your employer through a private attorney.
Did the 'resignation' you signed contain any waiver of potential claims against your employer?