I wonder how much they really know, GRRM has said they have an idea of how the overall story ends, but I doubt they know specifics on every character, hell I'm not sure GRRM knows all the specifics yet.
I think that before last year they just kind of knew the major plot points to ultimately end the characters arcs
But I read an interview that last year they sat down with GRRM and mapped out the paths for all the major characters so they could build an outline for the show
GRRM doesn't use detailed outlines in his writing, he just has certain plot points that he knows he is going to reach along the way. The journey to get there is built as he goes. So the type of stuff that he would be able to tell them is something like:
Dany joins back with the Dothraki ---> She gathers all the forces and sails back to Westeros ---> She battles Aegon for control of the Iron Throne --->She takes the throne as the Others are battling southward --->Dragon vs. Others battle ---> Dies in battle after being sacrificed by Jon Snow to form Lightbringer
But then all the details in between there....her encounters, her locations, her stops, they are made up along the way.
So Seasons 6 and 7 are going to be drastically different from the books, there doesn't seem any doubt about that. But the major plot moments for the characters should be close to the same.