Parallel Change (Or Expand, Migrate, and Contract)

The technique of Parallel Change was an original idea of Joshua Kerievsky. It was explained in 2010, in a talk called The Limited Red Society. InfoQ, Joshua Kerievsky, The Limited Red Society: https://www.infoq.com/presentations/The-Limited-Red-Society.

Parallel Change, also known as expand, migrate, and contract, is a refactor pattern to implement breaking changes safely (staying in the green). It consists of three steps: expand, migrate, and contract.

Expand

Introduce new functionality by adding new code instead of changing existing code. If you are expanding a class or an interface, introduce new methods instead of changing existing ones. If the behavior from the outside is similar and only the implementation changes, duplicate existing tests and point them to the new code, leaving the existing tests untouched. Make sure tests for existing code are still working.

Implement new functionality starting from the tests, either...