You'll have to rewrite the original GDFS. Sadly, most users never back it up before proceeding with whatever operation they intended to do in the first place. This is done through the "Read GDFS" button in SETool, btw . it will produce a .bin-file named with the phones IMEI and can be found in your SETool basedir. Take good care of it.
Now, lets say you didn't back up the phones GDFS and did something to screw it up. Then you have a problem, because rewriting it with another phones GDFS will screw the security units and the phone will be bricked. Generally a bad thing, right ?.
Luckily, there is a way to solve this and the procedure is as follows: