Here is my experience:
Last month I upgraded my Aa $95 card to an executive card so I could retain admirals club privileges. According to them, no hard pull and my card number and credit line stayed exactly the same. At the same time, I explained that I no longer needed the Prestige card after the benefit change ( I have a CSR and an Amex plat) but didn't want to outright cancel the card. They said at the one year anniversary which was on the 17 th I could convert it to whatever I want.
They called yesterday to remind me and today I converted to the no fee 2% cash back card. Again no hard pull but a new card number although the prestige will still work until I activate the new card when it arrives. In both cases the new account showed up on citi.com while I was still on the phone with the rep.
So to answer the question I don't believe so as there was no approval process or credit pull on either the upgrade or downgrade.