There's a trailer showing comparisons between original and the RTX version. It pretty much looks like a 20+ year old game with some improved lighting effects. They don't appear to have touched the level geometry or animations or anything like that...
It's a bit like what Half-Life: Source was to Half Life, though Quake II is far older at this point than half-Life was when its port was released.
Also, the headline is not really accurate saying "Quake II RTX will be free". You are getting a 3-level demo for free. Likely the same Quake II demo that's been available since the late 90s. You have to own a copy of the game if you want to play the whole thing. So, all you are really getting for free is the new renderer.
I suspect the main point is to provide source code to give developers an example of how they could incorporate RTX into their games, since the code for Quake II has been open-sourced for a number of years.
There ARE mods out there that make Quake II look not as old, with AI-assisted upscaled textures and surface improvements - the new renderer makes lighting much better, if you add better textures the game does look much better. Now, it would need more complex 3D models to really be made up to speed.