Look up this Document ID: KB46993 in McAfee's knowledgebase. It outlines using ePO and HIPS in a disk image.
You don't need to reinstall the agent. Just deleted the GUID as described in that article and the agents will generate a new GUID the next time they call into the ePO server. We had to do this very thing at one of our facilities.
I don't know of any way to get a list of machines sharing a GUID from the console or directly from the database. This is because the database only knows about whatever machine was last to call in with the GUID, they just keep renaming the same object.
I don't know what tools you have at your disposal.....I assume you know what the GUID is you're looking for? In our situation we used Zenworks to delete the registry value on whatever computer it identified as having the GUID in question. Then they generated a new GUID when they connected to the ePO server.
It might be easier to use a script to delete all the GUID's from your computers. Those that previously had a unique GUID should get matched to their previous object by MAC address. (Unless I'm mistaken ePO checks for a GUID match first and then MAC address)