Under Android, sharing a link from a mail does not allow a different sharing after first time. The first time I could choose a target (what to send to), but every time after that when I share, it just repeats the same sharing method (no choice) as the first time. Problem persists even after K-9 is restarted.
Thanks for reporting this. In future versions of K-9 Mail you won’t be able to select a default app when sharing a link (fixed here: https://github.com/k9mail/k-9/pull/5916), i.e. you will always be prompted to select an app.
In the meantime you should be able to remove the default app association by going to the App info screen of the app that is opened when you share a link from K-9 Mail. Select “Open by default”, then “Clear default preferences”. Depending on your Android version the names might be different, but the functionality should be there.
When sharing a link, make sure to select “Just once”, not “Always”. That will make sure you’ll be asked again which app to use the next time you share a link.