Sandman
In article <1h6ml9lr70cjtmt0f8lckalde0vfbkmck3@4ax.com>, Eric Stevens wrote:

Tony Cooper
I don't know how other newsreaders are configured, but changing the Subject line moves the post from the old thread's sequence position to a new place in mine, at least. It becomes a new thread.

Eric Stevens
Tools >Options >Display >Message list pane >Start a new thread when follow up subject changes

As I said elsewhere, in some newsreaders this is configurable. The default setting in Agent is for a new thread.

Sandman
Again, this is not actually starting a new thread. That option only tells Forte (which is one of the worst news clients ever to be made) to *display* posts that has changed the subject as a new thread, it doesn't *make* them new threads.