It will maintain the name it originally was created with. IMO, the easiest solution is to create a brand new Administrator account.
If you need any of the settings etc from his account then it's easiest to download and install the TakeOwnership script. Then you would TakeOwnership of his C:\User folder and TakeOwnership of your C:\User folder then copy his folders to yours.
At this point you can go back to User Account control and delete his account.
TakeOwnership can be found in our
Freeware DB in the File & Folder section.