Unlike its brother (Ubuntu’s Printer Configuration), Kubuntu Printer Configuration itself is quite obscure in setting Authentication/Credential. There isn’t a dialog for you to change your authentication, also there is no verify button like its brother. In order to change our credential, we have to change the Device URI. Don’t worry, after you apply it, the username and password will be hidden from the URI.
Syntax for SMB Device URI:
smb://USERNAME:PASSWORD@DOMAIN/SERVER/PRINTER
Sample Kubuntu Printer Configuration
Also your username and password must conform to URL encoding standard meaning, if your password contain says space or @, you have to replace it with %20 and %40 accordingly (h4x0r@ will be h4x0r%40 when you type in the device URI). There is no automatic conversion for you 
. More on special characters on URL encoding http://www.blooberry.com/indexdot/html/topics/urlencoding.htm
I agree this is a big usability issue. this becomes more problematic when your samba password changes. There is now no way to change that info. One has to manually delete the printer and add it again with the new password.