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.