As far as I know, you can rename your user in Win8.1, but the directory your profile lies in won't change to reflect the new user name! It will always stay the same as it was created.
Let's assume you named your user correctly ('John') in the first place, then handed your computer over to a friend named 'Heather' and she changes her profile name accordingly. From now on Windows will welcome her as 'Heather', but her user files will remain in the profile directory
The only way to circumvent this is to create a new user, transfer all files from the old user profile to the new one and then delete the old user profile.