The following is a direct quote from the Microsoft Community.
You cannot change a user account by just changing the name. Just changing the name will make a cosmetic change only and not affect the underlying files/folders in the profile.
1. From an account with administrative privileges, go to Control Panel>User Accounts and create the new Standard user account named the way you want and at least one extra Administrator account. Call the latter "CompAdmin" or "Tech" or the like.
2. Log into the new Standard user account once so the profile files/folders are created and then log into CompAdmin.
3. Now go to Control Panel>System>Advanced>User Profiles>Settings
4. You will see where you can copy user accounts. Copy the old account to the new one from there. Use the Browse to select your new account (C:\Users\newaccount).
5. Log into the new correctly named account and make sure all your stuff is there and that it works the way you want. When you are satisfied, you can log out and into CompAdmin and delete the old user account from Control Panel>User Accounts.
If you wish to log into your new, corrected account automatically, do this:
Start Orb>Search box>type: netplwiz [enter]
Click on Continue (or supply an administrator's password) when prompted by UAC
Uncheck the option "Users must enter a user name and password to use this computer". Select a user account to automatically log on by clicking on the desired account to highlight it and then hit OK. Enter the correct password for that user account (if there is one) when prompted. Leave it blank if there is no password (null).