Some uses of the tag myth seem to be about mythology, whereas other uses seem to be just saying something's not true. Should questions that are about mythology be tagged mythology instead, and ones where it's just about a false rumour have the tag removed?
I say go for it! There are such a small number of myth questions that a retag is easy. Votes on your question seem to agree.
Anything that aids in better categorizing questions is usually a good move.