Yes, so painting the property owner painting, say, a mustache on one of the characters, then leaving the name of the original artist on the piece, would be a clear violation, as that's an alteration. Removing the piece all together, after giving the artist 90 days notice to either move the piece (not really an option in terms of a mural) or destroy the piece at their expense would (probably) not be a violation unless you consider removing the totality of something an "alteration." That's certainly debatable.