As far as I know, there is no way to rename your PSN and keep your trophies/dlc
But if you're only worried about changing the PSN name, and keeping it attached to the email address attached to your current one, you can do this... but you need 3 email addresses to do it (possible 2 email solution below as well), and your trophies/DLC will still be with your old username...
1st email for your original PSN
2nd email for the new PSN you will register
3rd email for transferring
Instructions
1. Create new PSN name with 2nd email address
2. Log in to Playstation.com with the old PSN, and change the email to email #3
3. Log in to the new PSN and change email to email #1
It may be possible to do this using 2 emails addresses. You can try that by doing the following...
1. Log in to Playstation.com using the old PSN and change to email #2
2. Create new PSN with email #12
This may or may not work; depends on if Sony lets you immediately register a new PSN with your original email address.