You'd run into legal trouble if you start distributing the altered code. Otherwise who would even notice?
Section 117, Lease, Sale, or Other Transfer of Additional Copy or Adaptation, appears to address this issue: "Adaptations so prepared may be transferred only with the authorization of the copyright owner."
My question was poorly worded. I wasn't interested in an opinion about US law, but whether it is morally wrong for someone to hire a company to perform a modification for you.