Based on logic and common sense? What do you think their 400 developers assigned to it are doing exactly, farting in the wind?Based on what? they literally haven't said more than "it has decided to allocate additional time to both projects" which is meaningless in any interpretation.
When I say substantial rework, I do not mean that suddenly C77 will be a flying simulator or some ultimate life simulator. I mean that they are - probably - reworking, or trying to rework, some of the most criticised aspects, specifically the way civilian AI and spawning work. That is what I would expect. And aside from that, I expect the nextgen update to contain some new features/content.
Again, that seems like common sense. We'll see what they come up with of course.
But given that C77 was mostly developed in 4 years only, and was a completely new thing for CDP (new setting, new perspective, new combat system, stealth system, dialogue system, car driving...everything was different from Witcher), I would assume that some additional 18 months of development will result in substantial work.