You cannot change the name of your account once you have chosen it. You will have to make a new account.
If you are wondering, no you cannot transfer ANY content over to the new account. No money, no trophies, no items on PlayStation Home, nothing at all.
If you have downloaded games from the PSN store on the old account and still wish to play them on the new account, do not delete the old account from the system as the copyright licence will become void, meaning you can't use the games. So if you wish to keep playing the downloaded games on the new account, keep the old account on the system. Including any add ons like map packs for Call of Duty.
If you have bought any games that require an online network pass to play online, and you want to play these games online on the new account, you will have to purchase an online pass for the account from the PSN store for the selected game.
I hope this helps.