Refactoring

Refactoring is restructuring existing code without changing what it does — making it clearer, removing duplication, separating things that had become entangled. Behaviour before and after is identical by definition.

It is what keeps a codebase workable as it grows, and it depends on tests. Without them there is no way to demonstrate that behaviour is unchanged, so "refactoring" becomes rewriting with a nicer name and an unknown number of new bugs.

It is not a rewrite and should not be a project. Refactoring is done continuously in small steps alongside feature work; a proposal to stop delivering for two months to tidy up is a rewrite, and rewrites have a poor record.