You do not run a successful, stable software project over several years without some amount of automated testing. If several dozens of developers are working on the same code base, the need for test automation becomes even greater. After all, their changes might have unintended effects on other people's code, or on certain edge cases that will not be noticed until the changes go live — and maybe not even then.
Learn how we managed to move fast and create a new Symfony application without breaking our old legacy session handling. We write to our legacy session (which is file based) from our new project which uses PDO as the session storage.