If it is simply a title update, CDPR will make it optional.
They know how to handle PC games and releases, just look at their track record. They're the most pro consumer AAA developer/publisher.
This will be a seperate title, like borderlands GOTY and borderlands GOTY enhanced.
"The next generation edition of The Witcher 3: Wild Hunt will release as a standalone purchase for PC" Pretty much confirms it will be a seperate game within your library
