what would cause a catalytic converter to hollow out? must it be changed out?

You didn’t state year make or model. Now you need to find out if it has been hollowed out or has it been burned out, if it’s been burned out you can see melting of the honeycomb in the converter and that will tell you that something is wrong with either the basic engine or an over rich air fuel mixture. Now if it has been hollowed out you won’t see any honeycomb inside the converter as it has been removed completely. For the vehicle to pass emission testing (if required in your area) it must be replaced and if it was burned out the actual cause must be found or it will do it again.

You have a "brick" inside the catalytic converter that is what the exhaust gasses pass through on its way to go out the tailpipe. When the brick gets overheated from misfiring (the most common cause), internal engine coolant leaks, or burning oil, the brick melts and sometimes falls apart. Usually when this happens the muffler catches the debris and gets clogged up creating a restricted exhaust system and other driveability problems. If the muffler is not clogged I'd suspect the previous owner removed the catalytic converter, hollowed it out, and put it back on the vehicle to avoid the cost of replacing it. Which, by the way, is illegal. They either didn't realize or care about the check engine light being on constantly. Before you replace the catalytic converter make sure the engine is not misfiring, leaking coolant internally, or burning oil. Usually when a converter fails it's because of a problem with the engine. The new converter must be warmed up the first time after installation before the vehicle is driven as well. This is done by starting the engine and holding it at about 1,500 RPMs for 10 minutes. Then you'll be good to go. Hope that helps! Jim