You migrate existing subscribers using:
The <itunes:new-feed-url> tag. For more information, see RSS tags for Podcasts Connect.
Your web server. You can set your web server to return an HTTP 301 redirect response when it receives a request for the old feed. This response updates the iTunes Store with the new feed URL and your subscribers continue to receive new episodes automatically.
Be sure to maintain the <itunes:new-feed-url> tag and the HTTP 301 redirect response for at least four weeks to ensure that most subscribers have attempted to download your most recent episodes and received the new URL.