E3 2017 - Every game played on an Xbox One X will indeed play better, but native 4K resolution and other enhancements aren't guaranteed. Here's how Xbox One X works.
Microsoft has said from the start that developers will have the freedom to do whatever they want with Xbox One X's beefy hardware. But that kind of freedom means that there's no guarantee that all existing games will play at native 4K on the new Xbox One X. Developers have to upgrade their games with native 4K textures and add in other enhancements that tap the console's highly customized AMD SoC. Essentially Microsoft is handling the Xbox One X's game enhancements very similar to how Sony handled its Forward Compatibility upgrades on the PS4 Pro: developer participation isn't mandatory, but certain devs have partnered up to bring optimized upgrades to their games.
To bring native 4K textures and upgrades to Xbox One games, Microsoft has created the Xbox One X Enhancement Program. From now on Xbox One games supporting native 4K upgrades will be stamped with a little "4K Enhanced" on their boxes, similar to Sony's PS4 games. Developers must choose to opt-in on upgrading their games with native 4K textures, HDR, and other enhancements to tap Xbox One X's hardware and push it to the limit. These upgrades will be sent out via digital patches and updates, like PS4 Pro games. If a game isn't upgraded, it'll still natively tap the console's power, but it won't be supercharged in the same way that Forza Motorsport 7 is by hitting native 4K 60FPS on the system; the games will be upscaled to 4K UHDTVs, leverage super-sampling in 1080p HDTVs, frame rates in games with dynamic resolution will be tighter, and the games will utilize Xbox One X's 9GB of GDDR5 RAM game allocation to as a cache buffer to improve loading times and more.
And here's something quite interesting: just because an Xbox One game gets upgraded to run better on Xbox One X doesn't mean it'll automatically run in native 4K. Remember, Microsoft gives devs the choice to use the X's power as they see fit, and this doesn't always translate to bringing huge 4K textures over. "All the [titles we showed at E3] are going to get upgraded with true 4K upgrades. You're going to see that from us, but also from our third party partners," Microsoft exec Aaron Greenberg said during an interview with Geoff Keighley. "So you're going to have an incredible lineup of games. New games, plus a lot of games you already own are going to be upgraded with Xbox One X enhancements, and the large majority of those you can expect to be in 4K."