yes, this is a known bug:
http://kmeleon.sourceforge.net/bugs/viewbug.php?bugid=205
you can't 'remember' two (or more) sets of login info for a given site in a single profile. in order to change the info that is 'remembered', you have to first manually delete the previous entry in the login info file, which is in your profile directory, and is named with eight random characters and an '.s' extension.
not terribly convenient.