That error will actually clear up on it’s own after a few days.
When the license manager sees the license on a particular IP, it remembers that for a few days, but once Virtualmin has been removed from that IP, it’ll clear our of the license manager.
So, I’d suggest giving it a week or so… if that warning still hasn’t cleared up, file a support request (using the Support link above), and we can look into what’s going on there.