Not sure if this is possible, but curious… I am velocity Platinum and also have the AMEX velocity card.
I was wondering if you can cash in your free domestic flight with the card, and then upgrade to business using the Platinum upgrade… I am guessing not, but worth asking 
Not tried it, but I doubt it. The Amex free flight books into a deep discount economy class and last time I checked, the platinum upgrade was only applicable to full fare economy.
yes it looks like you’re right, I just looked up the rules of the upgrade and it’s freedom/flexi fares only… going to be slim chances of every being able to redeem those upgrades then!