Since the text of the question literally says "Is this a WW2 ring?", it probably isn't completely unreasonable for it to keep the WWII tag.
That being said, if it ever comes up again, I'd suggest just using "military" as the tag, rather than a specific war that the question itself openly speculates might not be the incorrect one.
If you are going to be bumping the question anyway for some reason (editing something or adding a bounty), that would indeed be a good time to also retag it, if you feel it could use it. Its generally not a huge issue if a question here or there gets bumped, as long as its not happening so often that users have a tough time finding the new questions through all the noise.