The Device Name is used as the key when importing devices. Therefore if you export your devices, change the device names and reimport, you will end up with a duplicated set of devices but with new names.
You probably want to try using the CatTools API instead. With this you can iterate the devices and update each one (so say you can do a replace string with new string, type of thing or append /prefix each existing name, etc.)
Here is a good starting point VBScript from our online help file which iterates devices in the database to perform an action on each one:
This help page list the available properties you can change using the API.
OK thanks Steve, there are some other examples there which also cover database edits, but as it turned out there weren't as many as I thought, so I did it manually.
Thanks for the update David.