When possible, we recommend archiving old forums rather than importing them into Discourse. Sometimes this isn’t possible, for various reasons, but getting a fresh start and moving into a new community platform without carrying all your old baggage across is often appealing.

Maybe your community needs a reboot.

But how do you

  • Keep the valuable information at old forums around without keeping that ancient forum software running, too, with all its future security vulnerabilities?
  • Avoid losing any Google juice for old existing links to good, useful community posts on your domain?

Source: https://meta.discourse.org/t/archive-an-old-forum-in-place-to-start-a-new-discourse-forum/13433