catalytic converter replacement

My catalytic converter is rattling, I tracked down the rattle and will try to fix that first.

But If the rattle for some reason can't be fixed I am thinking about just changing out the one catalytic converter that is rattling.

Will changing one side (passenger) have any negative effects or cause any problems? Also, will changing one side effect my emissions?

So many things "they" say to change both sides at the same time, does this apply to the catalytic converters too?

Thanks in advance!