Will importing to Fireside affect my subscribers?

No, nothing changes until you set up a 301 redirect from your old feed to your new Fireside feed. You can confidently import your podcast without any disruption. Once your new Fireside setup is ready, apply the redirect to move your subscribers and directories to the new feed. Until the redirect is active, your subscribers, Apple Podcasts, and other services won't be aware of the change, so make sure to implement it once you’ve posted new episodes on Fireside.