A:AnswerYes, this version of Assassin's Creed Valhalla will work on Xbox One! You do not have to activate or download the game immediately, and can wait to do so. :)
A:AnswerErik,
Microsoft basically stated that the way it works is that you get the upgrade on the same platform so if you buy digital, you get digital upgrade to XSX, if you buy physical, you can insert it into the XSX to play the new version of the game if that makes sense.
https://www.pocket-lint.com/games/news/xbox/152549-what-is-xbox-smart-delivery-how-does-it-work-and-which-games-are-compatible