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If you are offline for a long time, e.g. on an expedition in the wilderness, and your Premium expires, nothing happens.
Your app stays as it is with all Premium features. When you have subscribed Premium and you get back to the civilization, the app gets online and it finds out your Premium expired, it gives you a grace period long enough to re-subscribe comfortably.
If you decide to cancel the subscription, to subscribe to a lower plan or your one-time Premium expires, your data will remain preserved on the cloud. When you re-subscribe/purchase to Premium Gold, you can use the data again. At the moment there is no time limit for keeping your data safely stored.