When Microsoft ships the Xbox Series X later this year, it's going to have a feature called Smart Delivery. The idea is that you'll get the version of the game that corresponds to the hardware that you're using. If you don't upgrade to a Series X on day one, you don't have to worry about buying an Xbox One game, because if it supports Smart Delivery, it will automatically become an Xbox Series X game when you upgrade.
Obviously, not all game developers are on-board with this idea, but according to a report from VGC, Microsoft isn't letting them charge for the upgrade. In other words, developers can't make gamers pay for Smart Delivery. All they can do is sell an Xbox One version and an Xbox Series X version separately, similar to how things have been done in years past.
One example of this is NBA 2K21, which os offering a bundle that includes both games. It's worth noting, of course, that the Xbox One version of the game will still be playable on the Xbox Series X. In fact, everything that's playable on the Xbox One (including Xbox and Xbox 360 games) works on the Xbox Series X. It just won't be Xbox Series X Optimized.