As someone with admin rights I know of no way to do this cleanly with one action.
AFAIK the easiest way to do this is to NOT split the topic using the split topic feature as that will move the entire reply which then leaves one with the undoable problem of trying to insert a reply in between replies.
Instead
For the new topic
a. Create a new topic
b. Copy part of the reply into the new topic
c. If needed, edit the wording so it makes sense.
d. Change the owner of the topic
For the existing topic
a. Delete the part of the topic moved.
b. If needed, edit the wording so it makes sense.
This also causes more problems because it confuses users and sometimes the creator of the reply sees it as a violation of their personal content and leaves them with a sense of distrust.
The better solutions I found is to just use a person-to-person message (private message) and ask the user to consider splitting off part of the reply into a new topic and note that you can do it for them if they request and that they can then edit both items as necessary. If I do the change I suggest that they edit the title of the new topic because what they intended is not always what the guessed title suggest.
Thanks. Your technical solution looks like it would work fine - I might also add links between the two posts - but I agree that it’s annoying to have your words mangled by someone else. With the reply that led to my query here, I think the answer might be to rename the topic to be broad enough for both “sub-topics”, but in a more black-and-white case I’d likely go down the personal route, as you suggest.
It would be nice if the topic-splitting feature would (or could) leave the original post in the old topic so that it could be edited. That way the automatic messages (small action posts?) about topic splitting would make the situation clearer.
Think about it for a minute. Is it really topic splitting? I would think of it as reply splitting.
However if one is seeking a new feature then I think what would be better is for a feature where a moderator uses the wrench to select the reply then chooses an option that sends a canned person-to-person message asking the user to split the reply and then the canned reply includes some options right in the message to help the user split the reply.