Yeah, I've done this. It's frustrating and easy to burn out doing it because progress seems so arbitrary. Legacy upgrades are usually driven by large problems or the desire to add new features. Getting a grip on the code base while deflecting those desires can be hard.

This type of situation is usually a red flag that the company's management doesn't understand the value of maintaining software until the absolutely have to. That, in itself, is an indicator of what they think of their employees.