Note in the future, to not change the name of your administrator account.
It's a good idea to have a second backup administrator account anyway.
But if you really hate the old one.
First back up the entire disk using SuperDuper, CarbonCopyCloaner or another backup utility.
Create the new account.
Copy everything that you need from the old account to the new one.
Verify you didn't miss anything for about a week.
Then delete the old account after you store the...