[Flutter] How to fix the Google Play error: "This release cannot be completed because it does not allow existing users to upgrade..."

Google Play displayed the following warning, and I was unable to publish the app release.

This release cannot be published because it does not allow existing users to upgrade to the newly added App Bundle. 

Conclusion

Change this part of pubspec.yaml.

In my case, it was 1.0.4+1, but changing it to 1.0.4+2 allowed me to publish it without any problems.

I recommend checking the App Bundle Explorer and entering a version code that does not already exist there.

Recommended Books for Learning Flutter

I have compiled my personal recommendations for starting app development with Flutter.
I think it is a good idea to read new books for free on Kindle Unlimited.

Recommended Books

If you are starting now, you should avoid reading books written during the Flutter 1.x era.
The reason is that since Flutter 2.x, the "null-safety" specification was introduced, and the coding style has changed significantly.
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I started Flutter around the time 2.x was released, and I struggled quite a bit because the information was unreliable.
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For Beginners 【Free with Kindle Unlimited】

Let's make an app that works on your smartphone! Introduction to Practical Flutter from Zero ①: Let's try making a simple app
https://amzn.to/3yxutNM
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Detailed Flutter Explanation by a Beginner for Beginners
https://amzn.to/39YcpSK
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For Intermediate Users (Includes specific implementation methods and techniques like in-app purchases)

Introduction to Flutter Development for Practical Use
https://amzn.to/3kUXuLv