Change of citizenship for existing Nexus card:
Walked into Toronto Airport service center on 28th April afternoon with my brand new Canadian passport, 10 minutes wait to be called into the counter.

The Canadian officer was under the impression that it has to be done online, I said that it can be done online if it is just the passport but this is a change of citizenship which has to be done personally at a service center. The US officer agreed with me and they updated my details.

They kept my existing card and I had to pay USD 25 for a new card. It was kind of funny when the CBP officer took me through the US immigration lines to the US customs cashier for the payment. People in the lineups were staring at me thinking that I am probably being escorted to be deported or arrested. We had to wait for the cashier to come back from his break and had a pleasant chat about our families, home towns etc waiting for the cashier.

And the I just received the new card today, exactly a week after the update.