As far as I know it's perfectly legal to replace your catalytic converter once the original unit racks up 50 thousand miles. Cats are a wear item and eventually their insides get all caked up with grime that's been cooked on to the matrix over time. It's hot as hell inside there and all of the carbon particles and such get like, baked onto the inside surfaces. Eventually this coating gets thicker and eventually chokes off the flow through the cat.