Originally Posted by
flyerCO
I think you're confused.
Replace card just mails him a new Nexus card. Has nothing to do with anything else.. He'll get a Nexus card saying he's a Canadian citizen. Nothing else happens/changes.
His current card wont reflect his current citizenship. Presenting it can actually be a violation as it is no longer correct.
The card is fine to be used for NEXUS border crossing and other ID check. The former because the card is just a pointer to the NEXUS profile kept by CBP/CBSA.
Citizenship is generally not relevant for other uses with NEXUS card such as TSA ID check.