In my experience, any complex and widely used library is dead in the water without unit tests - if you make a change, you WILL break some use case, ending up in an endless cycle of fixing your changes based on user feedback, and souring your users on your library, who will refuse to update.

If you don't believe this, just write some (good) unit tests, then make the change - it's very very likely they will catch some weird corner case you've forgotten even existed.