If it's possible - I suppose it depends how many machines we're talking about - then if you can manually install the agent package from the server on these machines, they'll simply appear in ePO with the correct name. (You don't necessarily have to push the agent install from the ePO server.)
Have a look in the MA 46 Product Guide, specifically the "Installing on Windows" section - this describes what you need to do.
With regard to the existing entries with IP details, listed as Unmanaged - these can be safely deleted.
Great. Thanks. I had not actually seen that product guide before.