I know that some Stack Exchange site moderators have become Stack Exchange employees and then later left the company and gone back to being moderators on their individual sites. Sklivvz and Jeremy Banks spring to mind (although both of those are no longer moderators); probably there are others too. But those two both had dev roles in the company, not working as Community Managers. Other former CMs, such as Aarthi and rchern, were never (as far as I know; all before my time) site moderators, so didn't retain any diamonds after leaving the company.
It's going to be strange to see Jon Ericson, who's worked as a CM aka "super mod" for several years, acting once again as a normal site moderator. Less strange for former SE devs who didn't interact much with the community as part of their job for the company.
I wonder if this has happened before? Has any CM (specifically a CM, not other types of SE employee) left their SE job but retained their former moderator diamond?