I updated via keys yesterday to JPO using the reg hack. It changed my product code to KOR, but in this case, it isn't modifying the efs files. When you upgrade, it installs a multi CSC (i.e. multi localisation) ROM, and it sets the default CSC, and hence product code, to KOR. You can choose the correct CSC (mine was CPW, but you could just choose XEU for default European phone). When you do that, your product code changes back. Only thing to note, DOING THIS WILL TOTALLY WIPE THE PHONE AGAIN, so back up anything you need beforehand:
To change, go to the dialer and type:
*#272*IMEI#
where IMEI is your phones IMEI (this can be found on the box you got the phone in, or in the kies registry under HKEY_CURRENT_USER\Software\Samsung\Keis\DeviceDB\2 (the last number may be different for your PC). and look in the key called DEVCONINFO
G.