i'm not sure it will do what you want, 'cause i'm not sure what you want!
but from your description in your first post, it seems to me that that's what you want.
why don't you just try?
create a new user account with the new name. the corresponding folder in Documents and Settings will have the new name.
if everything is OK just copy the data over
(which may be problematic. some applications save data in subfolders of the Documents and Settings...thing might be broken. you might have to reinstall those applicatios, i dunno. maybe just running them will fix things - they'll create the new datafiles they need, but any settings/histories you may have might be lost. copying the files manually might work but then again might not.)
then delete the old user account!
(which BTW you can't do from Control Panel -> User Accounts. you have to go to Control Panel -> Administrative Tools -> Computer Management -> Local Users and Gropus)
if thigs don't work out, just remove the new account.
(BTW i've done this once: creating a new account, copying over the data files and also application files, worked OK for me).
as for the tip in that link...tried that psgetsid but didn't work ... i'm not familiar with DOS ...
anyway i would only try it after backing up all the data of the user's account.