"What can I do to change or motivate my low performers to be top performers?"
First off, it is important to note that you can't "make" anyone change, nor can you give someone desire to do so. The most effective thing you can do is create a culture where top performers want to thrive even more, mid performers want to become top performers, and low performers want to improve. You may also know what the low performers' talents are, then incorporate more opportunities to express that strength at work (i.e. more face time with customers, organizing a space, planning a project, etc).