Instructions
Scroll right to "Playstation Network" on your PS3 home screen and select "Sign In."
Scroll over to "Game," and scroll down to "Trophy Collection." Press "X" to access your trophy data. It will now automatically save itself to the Playstation Network.
Scroll over to "Users," select "Create New User" and type in a name.
Scroll to "Playstation Network" and select "Sign Up for Playstation Network." Press "X" to create a new account.
Type in the personal information required, along with your new Online ID. Highlight "Continue" and press "X."
Type in your date of birth and press "X" to continue. Press "X" to accept the Playstation Network terms of service. Now the name other gamers see online will be different, but all of your trophy data will be protected.