In our forum, due to the nature of subject matter, we have noticed that very often topics tend to split in such a way that a new subject matter emerges while entangled with the first one. This results in that it is really, really hard to split the thread into two, because it is possible that the discussion in several messages can be seen as belonging to both threads (old subject, new subject).
Now, to provide easier splitting of topics, I would suggest a feature: instead of simply moving some threads, you could “ghost copy” them, so that in the new thread there are copies of the original messages in old thread, but these are linked in such a fashion that liking/editing etc. one message affects both. This way they could be part of the new discussion that they seeded.
tl;dr Choose messages > instead of “Move to a (new) topic” you have “Copy to a (new) topic”. Would not double really so statistics would still be ok.