That's what I'm saying, Tiak. I was also under the impression that virii installed on other servers (non-owned) increase power rating, therefore, converting a private virii farm to an NPC server farm would effect power rating. I neglected to take into account, however, that the hardware factor to power rating would probably match if not exceed any power rating bonus due to installed virii. There also shouldn't be a way to track a server through IP change.
A deleted server which converted to an NPC server probably could generate a disinfect mission, but that would also depend on who it went to. I was assuming that it did not go to any of the four factions, but that may have been a bit naive and misleading--but that would preclude any possibility of generating disinfect missions and pretty well leave the server in limbo waiting for someone to purchase its IP. Deleted servers, therefore, should probably just vanish into the ether never to return.
As there are plenty of uses for unwanted servers, especially as private virii farms, this is not a critical issue, but it's fun to explore things, isn't it?