Note: Once you've completed the steps in this help doc to redirect your feed, you can start uploading new episodes to your Omny Studio account to update your feed.
Once you've imported your podcast to Omny Studio, you can easily migrate to Omny Studio without losing your existing subscribers. All you need to do is tell Apple Podcasts and the other podcast directories and apps to pull from your new RSS feeds. This is done by applying a re-direct from your old feed to your new feed.
Note: Although iTunes will normally pick up on changes within a few days, you should leave the old feed in place for a couple of weeks to ensure all subscribers are moved across.