I would like to pre-order a game for Xbox One in the Ubisoft store. The game will probably also be available for the upcoming console generation, but this is not yet an available option when pre-ordering. If I now pre-order the game for One and you can also pre-order it for the new Xbox over time, I can basically change it so that I get it for the new console or do I have to cancel the game and then order completely new?
Basically you should not pre-order games. Too many developers too often annoyed the players because the game was buggy or even barely playable on release.
To your actual question: Microsoft wants to introduce with the new console that you no longer have to buy games for both consoles if you later gamble on the new Xbox. So you buy for the One and can continue playing later on the Series X. Then it is called "smart delivery".
You have to check for yourself whether your game is one of them. Third-party developers can decide for themselves.
Ah okay, I didn't even know about the "Smart Delivery" system. But if I buy the game for the One, don't I have graphics and performance losses when I play it on the new Xbox afterwards?
Ultimately, it is up to the developer how much work he wants to put into the newer game version. But usually the games are graphically and technically much nicer on newer consoles. The difference between GTA 5 on the 360 and the One are also quite big.