In a post to his site Robert Basic makes an interesting suggestion about older codebases (legacy code) and how they should be handled. He suggests treating legacy code like 3rd party code.

Within the TDD community there’s an advice saying that we shouldn’t mock types we don’t own. I believe it is good advice and do my best to follow it. [...] This hidden advice is that we should create interfaces, clients, bridges, adapters between our application and the 3rd party code we use.

[...] What if we start looking at our legacy code the same way we look at the 3rd party code? This might be difficult to do, or even counterproductive, if the legacy code is in a maintenance-only mode, where we only fix bugs and tweak bits and pieces of it. But if we are writing new code that is (re)using legacy code, I believe we should look at legacy code the same way we look at 3rd party code, at least from the perspective of the new code.

He suggests that legacy code and new code should live in different parts of the application's structure and that, in order to use the legacy code, the new code should use interfaces to it rather than using it directly. He gives an example, showing the use of a User class from the legacy code and interfaces that could be used from the new code to work with it.