I think re-tagging should be considered in a case by case scenario, and should primarily consider whether this makes the site better. So consider these two scenarios:
Is a pure "identify this story" question made better by adding the tag once the story is identified? I'd argue no -- nobody will find anything interesting to answer while searching for this tag since, by definition, pure identification questions have nothing else left to answer.
Is a "hybrid" id question that asks meaningful followup questions about the story (which only make sense once identified) helped by tagging? I'd argue yes, since people searching for this tag will find something meaningful to answer.
In the case of the question that prompted this discussion, and which has an answer by me which was accepted, people looking for [dune] can find it and disagree with my answer (for the heartplugs) and maybe provide a better one.
Will the original asker be helped or not if someone provides a better answer, one that not only ids the movie (which we're sure by now was Dune) but also provides a better answer to the question of why were the heartplugs used in the first place? I think yes.
There's another question here: should hybrid/multiple questions allowed? I think not, because they bring this kind of problems, but I also think there's a grey area. In this particular case, I tried to convert it to a single question by changing the title to focus it on the heartplugs, but someone (NOT the original asker!) disagreed with this and rolled back my edit.
I think the question should either be split into its two components, and the one about the heartplug clearly tagged with [dune], or this one re-framed to be about the heartplugs.