If, for some reason, you need to change your Unreal Tournament 3 (III) serial number, I have an easy way to do it.

Here goes:

Browse to the Unreal Engine configuration file found at: C:UsersYOUR_USERMy DocumentsMy GamesUnreal Tournament 3UTGameConfigUTEngine.ini

Open the file, and near the end you will see this header:

[OnlineSubsystemGameSpy.OnlineSubsystemGameSpy]

Beneath the header lies your key, in encrypted format.

Remove the line that contains the key (in my case: EncryptedProductKey=001000000000208140157etc).

Restart Unreal Tournament III, and it will ask you for your key.

Enter your key again, and presto! You have changed your UT3 cdkey.