At the moment, in the Maintain -> Users -> Set Up User Security dialog, it is possible, without warning, to remove licenses from all Admin users.
Once this is done, no Admin user can log in, whether to re-issue him- or herself a license, or to upgrade someone else to Admin. The only ways this can be resolved are:
I suggest an addition to the dialog box, where, on clicking the "Save and Close" button, the software detects that, out of the users with Admin privileges, none of them have a license, and brings up an error message to that effect. Without, of course, saving the changes.
An alternative is, if no Admin user is selected, to make the "Save and Close" button grayed out and unclickable. The message box above, which starts with, "You have N licensed users...", could then have its content tweaked to tell the user that no Admin users are licensed, and that this must be fixed before the changes can be saved. I'm sure you can think of a good wording!