Is it hard to remove and change a catalytic converter on a 2004 nissan quest

That depends on available tools and talent .. Also, your zip code makes a difference - northern states naturally have more tendencies to develop rust on the undercarriage of a vehicle due to salt on roadways during winter weather - this will definitely make the work harder! .. Highly recommend having a 'muffler shop' install the exhaust parts for you .. That's what they do - it can be 'exhausting' work while lying in the driveway!