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You will need:
-FileZilla - Client Download
-PS3 FTP server
-SFO Editor ( You must login or register to view this content. ).

Instructions:

1. Run BlackBox on you Playstation 3.
2. Open up FileZila on your computer.
3. Go to hdd_0/games. Go to the BLUS of whatever game you want (DO THIS FOR ONLY LIFE WITH PLAYSTATION OR BLACKBOX). Other game usually don't work. BLUS for BLACKBOX is b100000 BLUS for Life with Playstation is NPIA000002.
4. Go to copy PARAM.SFO and copy it to your computer desktop.
5. Open up the SFO editor in your computer.
6. Edit it to whatever you want. Example: Modern Warfare 3. (Which is the one you are looking for.)
7. Save it and overwrite it.
8. Copy the file from your desktop back to the BLUS folder. It is recommended you copy it 2-3 times.
9. Restart your PS3.
10. Fool your friends!

Hope i helped!

Can you just copy the modern warfare 2 param sfo to ur computer with the gamesave utility and a USB?

Yes. But i don't think it will work. Is your Playstation 3 on 3.56 or 3.55 custom firmware?