Well, see if they are rattling. Jury is still out (mine is, at least) on whether cats are a time or mileage thing. My guess is it’s some complex algorithm that takes into account those two variables and several others we can’t think of. Mine were done (rattling) at under 65k miles, but can I say there was never any 100 Octane run through there? No. So I say if they rattle, replace them. But I get that you might have to “re-do” this later if you don’t do it now.