Before switching to a new device, you'll want to make sure you have already created a Calm account to ensure no data is lost. Once you've created a Calm account on your old phone, any session history or subscription information on that device will become synced to your Calm account so you can access it from other devices by simply logging in to that account.
Switching from Android to iOS
If your Calm subscription was purchased through your Google Play account, you will need to cancel your current Calm subscription within your Google Play account since your new phone no longer has Google Play on it. To do this, head to the Google Play Store and click your profile photo in the top right corner to open your account options. Next, scroll down to the Payments & Subscriptions section. If you have any trouble, please see this Google support article for more details: Cancel, pause, or change a subscription on Google Play
Switching from iOS to Android
If your Calm subscription was purchased through your App Store account, you will need to cancel your current Calm subscription within your App Store account because your new phone no longer has the Apple App Store on it. You can do so through The App Store on another device or by contacting Apple directly at https://getsupport.apple.com/.
Once your subscription expires, you'll need to renew it on your new device in order to keep enjoying Calm Premium.