You don’t have to do anything. K-9 Mail remains K-9 Mail. A mistake was made in the naming process. This will be fixed soon. So in F-Droid the following will be present:
K-9 MailThunderbirdThunderbird Beta
Thunderbird for Android is based on K-9 Mail, but has received a few changes here and there (e.g. the accent color is blue instead of pink). As mentioned above, K-9 Mail will remain and will probably continue to get the Beta & Stable versions in the version selector. Thunderbird has been split into two apps for this purpose. The settings should usually be able to be adopted from K-9 Mail and vise versa, because of the same codebase (but I haven’t tested it). In the end, it’s up to you whether you prefer the dog or the bird. :)
You don’t have to do anything. K-9 Mail remains K-9 Mail. A mistake was made in the naming process. This will be fixed soon. So in F-Droid the following will be present: K-9 Mail Thunderbird Thunderbird Beta
Thunderbird for Android is based on K-9 Mail, but has received a few changes here and there (e.g. the accent color is blue instead of pink). As mentioned above, K-9 Mail will remain and will probably continue to get the Beta & Stable versions in the version selector. Thunderbird has been split into two apps for this purpose. The settings should usually be able to be adopted from K-9 Mail and vise versa, because of the same codebase (but I haven’t tested it). In the end, it’s up to you whether you prefer the dog or the bird. :)
Edit: Added more info.