They are the same. It is the same app. They have announced that they will keep both K9 and Thunderbird in parallel, but the underlying program is the same - it will be only a cosmetic difference. This has to do with app IDs and user preferences, etc.
Here is the K9 Beta that is the same version as the new Thunderbird (beta) app. You are just switching from the beta back to the release.
Edit: sorry link was missing: https://github.com/thunderbird/thunderbird-android/releases/tag/K9MAIL_8_0b1
Not sure why people are downvoting you, you’re right.
Here’s the announcement from over 2 years ago.