I think this is the key description of 'Castle Rock';
"--a reimagining that explores the themes and worlds uniting the entire King canon, while brushing up against some of his most iconic and beloved stories"
If anyone's read it (and if you haven't here's a link), I think that the King story 'One For the Road' is a taste of what this will be.
It starts in a small bar, in a small town, that isn't Salem's Lot, but is not that far away. Everyone has heard of the town that died out, mysteriously, in the space of a few months, but no-one goes there and no-one wants to say the word "vampires" out loud, until one very cold, winter's night...
They're probably not going to retread the books, but they'll refer to the events from them and include some of the characters that appeared in them.