Will importing affect my subscribers?
No. In fact, nothing happens until you redirect your old feed to your new Fireside feed using a 301 Redirect. This means you can import your old podcast with complete confidence that nothing at all can go wrong. Once you're happy with your new setup, you can put the redirect in place to forward all of your subscribers to your podcast's new home.
Until you do that, none of your subscribers, Apple Podcasts, or any other app directories, services, or podcast clients will know about your new Fireside feed, so it's important to do once you've started posting new episodes to Fireside.