Start by doing a Reset on the Linksys router to wipe the ATT ADSL info including the PPPoE.
Reset will return the router to factory defaults and allow you to configure the router for the new service.
"Reset" involves pressing the Reset button for 20 seconds. You should not have to clone MAC addresses using this method.
Configure it using Linksys' instructions and you should be good to go.
Power down the cable modem, router and PCs.
Power up the cable modem, wait until almost all the lights are lit.
Power up the router, wait until all the lights are lit.
Power up the admin PC. Configure the network and wireless security.
Change the Admin password to something long and secure.
Document the password and settings for future reference.
The Comcast installer is not your LAN administrator so you cannot expect him/her to do anything with your router unless Comcast provided that equipment, so don't think badly of that installer.