It's not a super simple task, it's not as seamless as documentation makes it appear. While there is backwards compatibility, there is also isn't alot of backwards compatibility. Still have build certain systems and code base with UE5 in mind, as a lot of things are UE4 specific and very different in UE5, and then adapt that to the new engine codebase when you switch over.
A lot of differences in meshes, world building, lighting, water tech etc are also present in UE5, which requires manual work to adjust from UE4 standards to UE5 standards.
UE5 is no silver bullet, it's not going to fix every major issue with the game, but will improve workflow and offer a lot of stepping stones for future improvements, when UE5 matures over the coming years.