The purchase information of the premium plan is stored in iCloud.
If you plan to continue using the premium plan with the changed device, please turn on iCloud Drive on the new and old models. In case
Settings application> iCloud> iCloud Drive, turn on iCloud Drive and Shift Work Calendar.
Also, the Apple ID used in iCloud must be the same ID. Please continue to use the Apple ID that was used on the old device even with the new device.
You can also refer to this page.