Generally, forum discourse requires some form of comment freezing mechanism such as limiting editing to certain duration (e.g. it's 3 hours here) (or) as you mentioned change log as in Facebook, LinkedIn to ensure that the discourse stays meaningful i.e. someone shouldn't change their comment entirely and make the discussion meaningless.

But, in the case of collaborative story building may be we need both edit time limit and also change log? How about asking the user the 'reason for edit' and then displaying the same along with the old content when someone clicks 'edited' link on the comment.