The BuddyPress Group was set up long ago. I am setting up the bbPress plugin. There is a link at the bottom of the page you linked for migrating forums, but the page this points to seems a little outdated. It references BuddyPress 1.6.1, and we have version 9. It references Group Forums (Legacy) on the Components tab, but that is not present.
To be clear, BuddyPress 9.0.0 was installed long ago and has been used. It has a forum with posts. I have now installed bbPress 2.6.6, and I want to migrate the forum and posts from BuddyPress to bbPress.