You can do it, but you had better have a damn good reason for doing so. A friend of mine created his original account with a questionable username, one that PSN wasn't too fond of. It got closed down. He lost everything in terms of game saves and data associated with those games. Started over with new safer name. Contacted Sony about the problem, explained the situation, ate some crow, and wala, he got his original account reactivated with a new name. All original save data was again available to him(he was at like level 22 or something in GT5, and everything that went along with it).