Not so. You cannot patent any artistic work at all, for any reason. Patent can only be applied to novel concepts, processes or inventions capable of industrial application. Of the numerous things excluded from being patented, one is 'artistic works'. (Interestingly, another is computer software). In that case, copyright applies. You need not apply for copyright, merely assert it as your own. So George Lucas can make as many changes as he likes to the film - it's still his under any jurisdiction in the world.
In the specific case of the Mellotron, the name is a trade mark applied to a patented invention. Since Streetly don't own the name any more they cannot call their product a Mellotron any more than I could build a PC and put the name 'Dell' on it.
Mike