Windows Remote Desktop will not offer the option of editing the Logon Parameters if a password is stored in the .rdp file.
The user will see only "You can delete these credentials." ('delete' is underlined)
The 'password 51:' attribute line must be deleted from the .rdp file in order to enable editing of the stored credentials.
Then the user will see "You can edit or delete these credentials." ('edit' and 'delete' are underlined)