Is there anything in the announcement that says that people who installed K-9 mail would find that it has silently turned into Thunderbird? I'd assume this would involve a separate install, as it'll probably have a separate app ID indicating the Thunderbird connection.
In any case, I've had installed apps have the name change as a part of rebranding before. It's a little bit jarring sometimes, but name changes happen. They're usually small name changes, but I don't thing taking "K-9 mail is continuing development with a new name" as "app becoming something completely different" is really fair. Unless the functionality and UX completely changes, I don't think it's fair to read a name change as something so negative that needs to be "protected against".
In any case, I've had installed apps have the name change as a part of rebranding before. It's a little bit jarring sometimes, but name changes happen. They're usually small name changes, but I don't thing taking "K-9 mail is continuing development with a new name" as "app becoming something completely different" is really fair. Unless the functionality and UX completely changes, I don't think it's fair to read a name change as something so negative that needs to be "protected against".