Bit of a late reply, but it took me a while to figure this out as well.
To change the ID of device with ID 10.21.32.43 to mynewservername.mydomain.com, you go into the properties of the device. The URL will then be something like this:
zenossserver.mydomain.com:8080/zport/dmd/Devices/PATH_TO_DEVICE/XenApp/devices/10.21.32.43/devicedetail#deviceDetailNav:device_overview
Change the last bit (after the last /) of the URL to:
/renameDevice?newId=mynewservername.mydomain.com
you end up with:
zenossserver.mydomain.com:8080/zport/dmd/Devices/PATH_TO_DEVICE/XenApp/devices/10.21.32.43/renameDevice?newId=mynewservername.mydomain.com
Hope this helps :-)