If you're buying a new phone, it's possible to keep using Circuit. All you need to do is log in with the Apple ID or Google account you used to initially subscribe to Circuit. This is also possible if you change platforms (from iPhone to Android OR from Android to iPhone).
If you change platforms and want to change your payment method or cancel the subscription later, you'll need to do that on the platform you've originally subscribed on (Android or iOS). If you plan on selling your phone, we recommend you to cancel the subscription on the old platform and restart it on the new platform.