I would expect yes (or a printout of your previous reservation showing NRT-LAX). There are a number of connecting passengers in NRT who have to get boarding passes in NRT for onward travel - indeed, that's why UA and others have their transit desk just down the escalator from the security recheck. At worst, explain to them that your travel was originally NRT-LAX-DEN but that your flight was changed at the last minute because of fires in LA - and ask them to escort you to the transit desk if they don't believe you.