Our team went with a similar approach when refactoring Protobuf debug APIs (https://bughunters.google.com/blog/6405366705946624/fixing-d...). People were relying on debug output and trying to parse it, so in the new implementation we threw up big warning flags and made the output unstable so that you couldn't make the mistake.
The key lesson to take away is: if you want something to be an implementation detail, make sure to have multiple differing implementations :)
The key lesson to take away is: if you want something to be an implementation detail, make sure to have multiple differing implementations :)