What can and can not be edited in old posts?
I am quite confused as to what we can or cannot edit in old (already indexed) posts. In numerous posts I have read that it is good to go back and improve old posts, perhaps by adding images or adding new information. Yet, in other posts, I hear warnings that changing old posts can be detrimental to rankings.
Obviously, I have missed the point somewhere. Can someone explain what can be changed without causing issues with rankings? I've asked and been told not to change the title or the permalink. Do things such as categories or keywords or SEO description cause any disruption?
Thanks in advance.