We can't stress having proper documentation enough. Take any and all paperwork regarding the catalytic converter purchase and installation with you to the smog station when it comes time to get your vehicle smog inspected. The Bureau of Automotive Repairs is enforcing CAT regulations so tightly these days that is has smog technicians crossing their T's and dotting their I's like never before.
Perhaps the reason the smog technician needed to fail your vehicle during the previous smog check was because he/she identified the vehicle's CAT as having been on a Catalytic Converter Rescind list. If this was the case, the CAT would have been able to pass the smog test only if it was purchased and installed prior to the date it was rescinded. Documentation proving this is key.
Find your documents, keep them in a safe place, and present them to the smog technician when the time comes. He/she should pass the catalytic converter visual inspection if the CAT as purchased and installed prior to its rescind date... which by the way, you can find out yourself through the California Air Resource Board aftermarket catalytic converter lookup
website. There was a surge in rescinded catalytic converters in January of 2012.
Often even with receipt/documentation for CAT purchase and installation, if there is the slightest doubt about the validity of the documents, the technician will refuse to inspect the vehicle and will refer the owner to the smog check referee's office. One missed catalytic converter may cost a smog station there STAR certification, which for a test only type smog station means the difference between staying in business and not.