If one's complex paper-handling and mark-delivery robot is working fine, why change the software inside it? In other words; software for robots can have bugs j

If one's complex paper-handling and mark-delivery robot is working fine, why change the software inside it? In other words; software for robots can have bugs just like software for anything else, so why take the risk if the thing's doing what you need it to and the replacement software doesn't solve any problems you have?