Discussions do not belong to tags, tags are merely assigned to discussions. When a tag is deleted, nothing happens to the discussion except that they lose that particular tag.
If you want to "move" discussions to another tag you need to do so before deleting the tag. It can still be done afterwards but of course you will no longer be able to know which discussions had the now-deleted tag.
You could use Mass Actions to assign a new tag to all discussions that have a specific tag, and then delete the old redundant tag.
