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Just open and edit the "Subject" of the first post, and in case of other people responding, also the "subjects" of other posts, if you need to.
We don't have such a function, because each user can change the subject in their response, i.e. the "topic" will change automatically in his response.
As a moderator, I've changed your original topic by adding the (x) to it.
In my response, I've changed the topic by changing the (x) to (y) and adding the "(x becomes y)"
This way people can adjust their topics, and if the discussion moved to something else, the message can reflect that.
