There's really no good way to handle this.
The way that's arguably most in-line with the SE design philosophy is to downvote wrong answers (even answers that are wrong through no fault of the answerer) and upvote correct answers.
The problems with this are:
- New answers to old questions typically, though with exceptions, get less attention; it's much harder for a new question to float to the top of the page. This leaves us in the situation where the "right" answer and the "best" answer (according to community votes) aren't necessarily the same, making the whole site a more complicated thing to use and rather undermining its purpose
- We can't do anything about the acceptance mark. In some cases this isn't a problem, because some of our long-time users are still active and open to moving the tick to the better answer. But in other cases (such as the one you linked to), we wouldn't expect a response
The other way would be to intentionally post duplicate questions, answer them with the new canon answers, and then close the older questions as dupes of the newer.
This has the advantage of more effectively promoting the new content, but isn't without problems; in addition to the ones I've railed about before:
- Some people object to the practice of closing old questions as dupes of newer questions. If we were to do this en masse, I expect a lot of arguments in meta
- It sets an uncomfortable precedent that may get out of hand; this is admittedly a slippery slope argument, but then building on precedent is most of what Meta is about
The final way would be to edit the existing answers to say something like "As of the Great Disneyfication, this answer is no longer correct. See [link to correct answer] for the correct answer."
This is a nice middle-ground, but will result in a lot of edit spam, and probably some backlash from people who object to their answers being devalued
My philosophical inclination is to go with the first option I posted; I've whinged mightily against the second option previously on Meta, and I stand by those arguments.
However, in this specific case, of Star Wars Disney canon versus Legends, I think it would be more effective to ask and answer new questions, and close the older ones. My biggest reason for saying this is in response to a point I made in my Cursed Child meta answer:
It stratifies the canon in a way I'm not comfortable with (in most cases). There's little intuitive reason why a question like "Why is X a thing in Harry Potter" and "Why is X a thing in Harry Potter and the Cursed Child" should have different answers
After the canon reorganization, there really is a stratification of the canon, imposed by Disney. So much of the basis for our older answers is now out the window that I think it will be simpler and more effective to "pull a Disney" and declare many of our old Star Wars questions to be non-canon.
I'm not opposed to any of my other suggestions being implemented, and I feel really icky about even suggesting this one, but I think it will ultimately be more effective in helping us fulfil our purpose of being a useful repository of facts.