Usually when people edit messages there's a notation that it was edited.  Some even note the number of times something has been edited, and by whom, including when mods/admin edit it.  I forget what options discourse has for that.  But also, when you are quoting someone on a forum, if they go back and change their message, i dont recall ever seeing a forum where it auto-updated the quote replies.  So the person's original message will still show, however that does cause a bit of confusion when a new reader comes to the thread and sees a quote that doesn't match what they see in that person's post.  There was a forum somewhere that the edit message would say "This post was edited by ${user} after replies by ${user2}, ${user3}, ${user4}" which helped eliminate that... but I believe that may have been something custom that the admin put in place. 
Also there's usually a time limit in place for edits... I've seen it as low as 30 seconds on some forums which negates some of this issue.