Legal Question in Intellectual Property in California

If indeed there is a breach of contract, I'd say you should consult with a local attorney who understands and practices both (1) contract litigation, and (2) intellectual property, probably with emphasis on #1 but some knowledge of IP law (copyrights) would be needed. The attorney should advise you whether to sue immediately (whether you have a sufficient claim for money damages, whether you need an injunction to stop publication, etc.) or whether negotiation spearheaded by an attorney would be wiser at this stage of the game. By the way, there is also a possibility that the magazine is entirely within its rights; I couldn't give an opinion without more information re the contract terms and exactly what the magazine intends to publish.

What you don't mention with the commissioning is how was the agreement structured absent the sale of the publishing rights to the magazine. I agree with Mr. Whipple about emphasis on number 1; contract litigation because everything is initially dependent on the Contract. Which is why in any contract litigation I want to see the contract first.

About free work, are you providing sketches as well when you don't have to pursuant to the agreement? You could do things if that is the case to protect your interests. Otherwise even if you sold exclusive (did you make it non-exclusive?) publishing rights to someone for the final work they can still infringe it by substantially altering it.