On the QaFoo blog there's a new post sharing some ideas on how you can add new code to a legacy application and ship it successfully without too much interruption to the current code.

Usually the problems software needs to solve get more complex over time. As the software itself needs to model this increased complexity it is often necessary to replace entire subsystems with more efficient or flexible solutions. Instead of starting from scratch whenever this happens (often!), a better solution is to refactor the existing code and therefore reducing the risk of losing existing business rules and knowledge.

[...] Instead of introducing a long running branch in your version control system (VCS) where you spend days and months of refactoring, you instead introduce an abstraction in your code-base and implement the branching part by selecting different implementations of this abstraction at runtime.

They then give a few examples of methods that can be use to get the new code in:

• Replacing the Backend in a CMS
• Rewriting a submodule without changing public API
• Github reimplements Merge button

The final point is broken down into the process they recommend including the refactor of the current code, starting in on the new implementation and deleting the old code.