Bethesda previously announced that Doom Eternal and The Elder Scrolls Online would be among the first of its games to make the generational leap, and the studio detailed the process for the original in a recent blog post. Simply put, Doom Eternal will be playable through backwards compatibility when the next-gen consoles arrive, but the visual enhancements you'd expect from a next-gen game won't come into play until later, and Bethesda hasn't committed to a date.
"We have not announced when these upgrades will be available," the studio says. "Upgrades will apply to both physical and digital versions of the game, but please note that physical versions will not be able to upgrade on the digital-only versions of the next-generation console." The studio added that, "When we know the timing for the availability of the upgrade, we will let everyone know."
Xbox One copies will transfer to Xbox Series X, PS4 copies will transfer to PS5, and both versions will need the original disc when upgrading physical copies. In other words, if you get a digital edition PS5 or an Xbox Series S, you won't be able to upgrade from a physical copy of Doom Eternal, which is standard for the new generation.
The good news is that every part of this upgrade process is free. Physical copies can be upgraded on next-gen consoles with disc drives, and digital copies only require a new download. Any extra content and DLC purchased for Doom Eternal will also transfer to new consoles with no charge, so all you have to lose is a bit of time.