Your history and unfinished edits are stored on your device. The app has no feature for carrying them over to a new device through an account or the cloud. They may be moved along with a device backup or data transfer, but this isn't guaranteed, so please be aware of this.
Before changing devices, use "Save" to save any images you want to keep to the "Pictures" folder on your device. Saved images can be moved to your new device when you transfer your device's data.
*You can get Premium back by tapping "Restore Subscription" on your new device.