I'll admit I don't know much about Halloween posters, but that one seems to use the same image of Michael as used on the Halloween 4 and 5 posters but the credit block from the original Halloween poster.
If it is legit I would be doubtful if it was an original release poster. Having said that if it was a re-release poster I would have expected to seen some text referring to the fact that it is a re-release or special event.