Going from their existing RED engine to Unreal is basically the same idea. Almost nothing from the original Cyberpunk game is going to be easily translated to the new platform. I think CDPR just set their development timeline back by at least 3 years.
Given how massive their game is, I'm doubtful. So much of what they did in the first game will have to be rebuilt. Compared to just reusing most of the original assets and code, this sounds like a lot more work.
Maybe, it might also be easier to reuse portions of the engine in Unreal Engine while using parts of Unreal (like its rendering engine) than you think though. Assets largely I'd expect to be portable or at least comvertable with a custom asset loader.
I'm talking a little out of my ass though, and neither of us is familiar with the code. Point being though, it's a little different moving engines than rewriting a complicated web server (a project I have been a part of and would not recommend).