That old software is full bugs, they've just figured out to work with them.
If a customer doesn't change their workflow, they aren't going to suddenly stumble over an old bug.
But if you keep pushing changes, eventually you'll introduce a new bug that does affect them.
There's no way around it: The more you change software, the higher the risk of introducing bugs.
That applies even if the changes are just bug fixes for old bugs. Every time you fix a bug that I wasn't affected by, you risk breaking something that I do use.