Log in to the Assassin's Creed Valhalla owner Microsoft account on your new console to activate Smart Delivery.
Seriously, that's it. Smart Delivery essentially checks your account for all compatible licenses, and it looks like anything that supports the service in your library will be available for download on your new Xbox Series X | S. It really is as easy as logging into your account, then going to your games and library and installing Assassin's Creed Valhalla. If you have the disc version of the game, the process is just as simple: log into your account to which the license is attached and insert the disc.
Now in theory Ubisoft Connect might be running an additional ownership check, but after booting and restarting the game a few times, I've never seen anything like this pop up. It might be the faster load time for Assassin's Creed Valhalla blowing the prompt, but I've never seen it. Now, it goes without saying that you will need to be connected to the Internet for Smart Delivery to work. You can obviously bypass it with the disc, but you won't import your save files and the game will be recognized as a new license if you are not connected to the internet on your Xbox Series X.
I can confirm that logging out after verification is not always pleasant. Sometimes the game will start, other times you will be presented with the message “you must be online to play this title”. Odd for a single-player game that's largely offline, but it could be a background glitch stemming from Ubisoft Connect's own verification (again, assuming it's even there). Also, if you own the physical version of the game, you will need to keep the disc inserted to play. So if you bought an Xbox Series S or sold your disc, you won't be able to play Assassin's Creed Valhalla period on your new console.
There is good news though: all of your DLC and purchases will transfer between consoles, and you can even go back and play Assassin's Creed Valhalla on your old Xbox without hassle. Naturally, you cannot transfer between Xbox and PlayStation as they are different platforms from different companies and Smart Delivery is not supported on Sony devices. That said, if you buy Valhalla today on your Xbox One and upgrade to an X or S series later, you will be able to defer your purchase (without physical copies and S series).
