Sorry, but I understood you incorrectly first time and another Engineer was right about that Purging Maintenance can't change IsManaged status from 1 to 0. Actually Purging Maintenance can retire only computers with "managed 1" and that have not checked in for a N time.
Computers with agent installed have "IsManaged = 1"
Computers without agent (uninstalled, discovered, imported,etc) have "Is Managed = 0"
Could you please clarify the current situation on your envirment. Do you still have computers with "Is Managed = 1" wihich are not using anymore and you want to purge them, but leave the history? In this case "Retire the computers. All data will remain in the database. But these computers will no longer be managed by Notification server" purging option could help. Also please clarify about:
If I change "Managed" to 1, it comes back with about 18k and our licensing shows just over 12k which is the more accurate number.
Thanks,
AN