Hello @milnem and welcome to the Community. As you know, Fitbit's Cardio Fitness Score is its estimate of your vO2 max. It will keep working if/when you add your Versa 3 to your account. I've used multiple devices over the years. Works seamlessly.
Laurie | Maryland
Sense 2, Luxe, Aria 2 | iOS | Mac OS
Take a look at the Fitbit help site for further assistance and information.