The easiest method I've found is to first change the passwords directly, and then restart PrintingServices. The passwords are stored in a root-owned file at /etc » cups » printers.conf, in a URL for each printer:
DeviceURI smb://USERNAME:password@Workgroup/path/to/printer
You can easily edit this file in Terminal.app with sudo vi /etc/cups/printers.conf (Type :1:$s/old_pword/new_pword to change them all at once!). Save the file, then restart CUPS: sudo SystemStarter restart PrintingServices.

