Legal Question in Employment Law in California

Yes it is entirely legal unless you have a contract that both states you will be allowed to work at home AND that you have employment for a specific amount of time unless fired for cause. Otherwise you are an at will employee and can be fired for any reason or even none at all, just as you can quit for any reason or none at all. That carries with it the lesser included right of your employer to change any term or condition of your employment at any time. You can then quit if you don't like it or they can fire you if you don't comply. The only thing in your favor in this is that such a radical change in your terms of employment would probably allow you to quit and still collect unemployment, but you should definitely check with EDD before doing that.