I’d say update the post and leave the old link in the text for posterity.
Yep, make a note in the post body about the change, so that people have context for comments on the old article
This depends on how different the new article is
Edit the post to say better link in comments and put the new one there. Leave the original alone.
I’d say since Lemmy supports link posts with post text, you could also just put the link in the post text itself rather than comment it
I think you should link the better article in the comments or in an appendix to the post text.
A post that is about an article should remain pointed at that article, in my opinion. It’s a matter of referential integrity.
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I would just add the better link to the post. for example
http:originallink/
edit: here is a better link http;//betterlink