I will add some example posts to this conversation and then use them to make screenshots to show the process.
First you selected split from here for the first post you want to split out.

Then go to the last post you want to split out and select split to here

Then enter the title for the new discussion

Once you have the new discussionyou can then edit its tags and tag it differently from the original discussion.
As you see I have separated out posts 2 and 3 from this discussion into a new discussion and posts 1 and 4 remain behind.
The split is irreversible but you can cancel it at any stage in the process until you hit Split on the final dialogue.
If any of you want us to split off part of a discussion then contact @Support or me or Stevie