That idiom essentially means it is a mistake to try to improve something that works.
I will admitt to jackin a line from the good ole' doctor, Dr Phil. Whenever someone continues to rally around a series of behaviors that everyone around them can see are mistakes, Dr. Phil asks this question: How is that working for you? I know---I know. Many people think Dr. Phil has given psychologists a bad name. But say what you will, that line works every time!
I use that line almost everyday of my life. I get phone calls, emails, text message, and even letters in the mail (yes---people still mail letters) from people who tell me why they engage in behaviors that even Stevie Wonder can see ISN'T WORKING. Yes--- it is a mistake to try and improve upon somethinng that is working very well for you. But it is an even bigger mistake to hold on to something that is broken with the hope that some how---some way, it will magically improve its self.
More to the point---look around you. Look at your job, your spouse, your friends, your thoughts, your words, and your actions. I got one question for you. How is that working for you?!.