Usual disclaimer I am not an expert.
I can not imagine anyone throwing away an engine and starting from scratch. That is way too much work. One of the main things you would change is how scenes are drawn. Every few years or so the method you use to drawn an object to the screen changes as videocard hardware evolves.
Simple examples:
ENGINE #1 - You send rotated vertices, pre calced lighting, and a texture to the video card.
ENGINE #2 - You send the vertices and texture. Videocard does the rest.
ENGINE #3 - You send a memory pointer to an array in PC memory. Videocard pulls data in on its own and draws object.
ENGINE #4 - You store all data on Videocard but it runs like Engine #3.
ENGINE #5 - Data on videocard can be manipulated for animation. Includes LERP between keyframes etc.
Other examples:
ENGINE #1 - Uses one texture at a time.
ENGINE #2 - Uses two textures at a time, one is diffuse, one is static shadowmap.
ENGINE #2.5 - Adds procedural created textures on the fly.
ENGINE #3 - Version 1.0 of shaders is used where you can now control per pixel drawing. Bump mapping begins. Heightmaps are used. Realtime lighting begins to be used on Videocards but is very slow to process. ID creates 1st game to use it DOOM3 creating a dark environment as each light requires a full scene render.
ENGINE #4 - Shader model 2.0 is used. Normal maps are added for better per pixel lighting. Specular maps are added for shiny surface light reflections.
All of these videocard updates could create large code changes. You may need new data structures. They may be stored differently. When should they be sent to the videocard, etc. The tools to create maps/levels may run completely different and need to be rewritten or modified.
This is all for PC programming which is somewhat straight forward using predefined APIs such as OPENGL and DIRECTX. Using languages like C++ that have been optimised by Microsoft, etc for years.
This is why premade engines like Unreal, Unity, Crytech, IdTech are the way to go for most games. All of the hard time consuming videocard work has been done for you. If I was Relic I would lean towards Unreal as it looks the best. However, as Tric pointed out, PUBG dont run so good
TL;DR
The main point of Relic updating its engine is that consoles need very specific code written for them. So maybe this is an indicator of testing the console arena. PC Games are slow because the author has no idea what hardware you have. So they write to generic libraries OpenGL, DirectX, OpenAL, etc. Consoles run very well because they know exactly what hardware you are running. The code can be optimized down to the hardware register level increasing speed 2x or 3x over generic PC libraries. Throw a Playstation into the mix and it gets even harder because the CPU structure is completely different and there are specific cases where you can pipeline multiple commands simultaneously. Console require a much deeper understanding of the hardware.