If I upgrade from my Xbox One to the Xbox series S, will I need to re buy digital versions...

I am thinking of buying hogwarts legacy as a digital game but it is sold out for Xbox one so should I buy the Series X game and put the code in the Xbox one Store?

No. The series X/S version of the game will only work on the Series X or S.

The only way that would work is if it's a code for the digital deluxe version of the game (which comes with both the series x/s and xbox one versions of the game)

From what I can tell, to access the Series X version of the game, you need the Cross-Gen bundle...once you buy that, you should then be able to download and install the Series X version. Without the cross-gen bundle it will, presumably, just install the XB1 version

From the Store page:

"For digital owners of the Call of Duty®: Black Ops Cold War Standard Edition, this upgrade will unlock the Call of Duty®: Black Ops Cold War Cross-Gen Bundle, giving access to the Xbox Series X|S version of the game. After upgrade purchase, search in the Xbox store for the Call of Duty®: Black Ops Cold War - Cross-Gen Bundle to install the Xbox Series X|S version of the game."

The digital deluxe edition will only work on the Series S/X. You would need to buy the Xbox One version of the game.

If you buy the digital deluxe edition, it includes both the One version and the Series S/X version.

Otherwise, you would need to buy both versions since the Series S/X version won't work on a One.

no. I believe you still need to get the digital version of the game if you're referring to Xbox series S or all digital Xbox one S