Continuing the discussion from Feedback for Forums Redesign Sep 2018:
Great question. How do you take an existing discussion and start a new, fresh discussion, so you don’t derail the first? Reply as a New Topic.
To reply as a new topic, click on the timestamp icon in the top right corner of the post to load the Share pop-up.
From the Share pop-up, click on + New Topic.
Give your new topic a title and add your message.
Click Create Topic
Notice that your new message includes an auto-generated link back to the source post.
And the post that started it all will get updated with a link to the new reply.
Pretty cool!