Non-degree programs are usually reserved for people who already have a bachelor's degree but want to pay a university to take some courses without planning to claim a degree at the end. Sometimes people do this out of sheer interest, but in other cases they might be seeking to bolster their transcript before seeking graduate admission in a subject very different from their bachelor's degree.
As such, simply stopping taking courses is a fine way to unofficially convert a degree into a non-degree program. If you don't finish, you don't get the degree. At that point, the only difference I can think of surrounds what happens when you have taken too many credit hours but still haven't graduated. Many universities have some thresholds in place after which incentives start to push students towards graduation. That might be more expensive tuition or hard caps on the number of hours they can take without graduating. Given all this, I don't think many universities would allow a student to convert their degree program into a non-degree program.