What is your browser? With Firefox there is a password manager that allows
you to deal with one password at a time, rather than losing them all you can
remove as many or as few as you want.
But, it does acts the same way as autocomplete..removing it does not remove
the password from the web site, it just no longer remembers it when you want
to log-in.
To change the user name and passwords for web sites, you'd need to go to
that website and see if there is an option there for changing those.
XP, or your browser only remembers them for you, they have nothing to do
with creating, or changing them for websites.