The igroup information is storage internally and is available through the CLI (igroup command) and the API calls.
There is no other supported or documented method to access this information.
> Is it also true, that when a head is upgrade from a 3020 to 3040
Unfortunately this was true but has been fixed with the 7.2.4 release.
Starting with 7.2.4 the serial number will not change when the controller is updated.
> I've noticed that the LUN serial numbers are different on our SnapMirror copies at our DR site, .....
> Why do LUN serial numbers change?
This is by design to provide data integrety. When ever LUNs change "locale" (volume, storage system, etc...) the serial numbers are changed. There is currently no method to retain the original serial numbers. It is possible to set the serial #'s to the original values using the "lun serial" command or api.