I went to upgrade my DCS app from 3.1 to 4.0 after upgrading I had no premium app so in order to use it I had to buy the premium app again. from the Google play STORE, I’m wondering if this is correct
No, it is not. The Standard and Premium upgrades are tied to your Google account. If you sign into multiple devices with the same Google account, your DCS APP upgrades will follow you on those devices. If you sign into a device with a different Google account that has never purchased either of the app upgrades, you will need to buy them again.