No, it is not legal. There was a contract, pursuant to which the art was created: the understanding (even if it was an oral, or unwritten one) pursuant to which you commissioned the work and the artist created it. That agreement is enforceable and binding; the artist cannot legally change the terms under which he did the work. That does not mean he won't try, or won't file a lawsuit--since courts don't "prescreen" lawsuits for validity, even weak or bad cases can be filed, forcing you to respond--but based on what you write, it would appear that the artist is bound to what he'd agreed to before doing the work.