On some boards:
It is possible to move all posts after the selected post into a new thread. The moderator (or admin) would have to give the new topic name, but then the selected post becomes the opening post of the new thread. It may also be possible to move additional selected posts with the new OP, depending on what tools the board has.
For example, I'll make a list of "O"s for "On Topic posts" and "T"s for "Tangent"
O
O
O
O
T <>
T <>
T <>
O
O
T <>
O
Now a moderator could select splitting the thread, and move all of the T posts to the new topic, leaving the Os in the original topic.
This is functionality that would have to be intentionally built into the forums to do it.
It wouldn't allow you to move posts from 1 existing thread to another existing thread unless a "move posts" functionality were also implemented.