Suppose we had a discovery rule that discovered devices in the IP range from 192.168.1.1 to 192.168.1.20. During its operation it discovered a device at 192.168.1.15. We then changed that discovery rule to scan the range from 192.168.1.1 to 192.168.1.10. The device at 192.168.1.15, however, will remain in "Monitoring" -> "Discovery" history for that rule forever.