Have Bethesda had to alter Far Harbor's content so the new mode will work or was the new Survival mode already in the planning before they started work on the DLC, that is what I want to know.
Will you go to Far Harbor in the new mode?
It'll easily be compatible, that's nothing to worry about.
According to that gameinformer interview, the survival overhaul and a ton of other DLC ideas were born from an internal gamejam they did the first week the game came out.
I have a feeling you will be able to craft a portable sleeping bag and maybe be able to make a camp fire to cook food and maybe purify water.