Legal Question in Employment Law in California

The boss is mad you're rescinding your acceptance of the shift? Anything wrong? Yeah, you're in breach.

CA law presumes you're at-will. You can be terminated, demoted, etc. at any time without warning and for no reason at all because it is at-will employment. What you have is a contract to work for pay, even if its not in writing. What you did here is accept his additional offer to work more and now you've tried to rescind and are in breach. You ought to "try it out" as you said, and presumably collect any overtime if you can, before you get terminated and they can hire someone else.

Note people often think if they just show up on time and work they cannot be fired for some trivial reason. Well life can be unfair.

Your supervisor did not do anything against the law, and he did not do anything for which you can take any action against him. If you and your employer cannot agree upon your work schedule, then your employer has the right to terminate your employment and find someone that suits his needs -- just like you have the right to quit after finding another job elsewhere.