Edit user account always shows current user in username even if editing another user

Logged in as admin - edit another user's user account e.g. to change password, block/unblock etc. the username field is always filled with 'admin' instead of the actual user's name so you have to change it before saving otherwise get an error that 'admin' is already in use.
Even if you do change it it's too easy to make a typo and accidentally change the real user's id so he/she can't log in.
I would have thought that changing username is done fairly rarely and could cause major problems if accidentally edited so that it would be better to have a checkbox associated with username to confirm that it is really to be changed.
I'm not sure if the version number I've selected is correct - it's the user maintenance included with Drupal 5.6 (I'm a Drupal newbie)