Demo is not transferable to the full game. You lose all progress and start over.
Relating to that... I read an interesting article that talked about how the game has Denuvo, but it was "cracked" using the demo's executable. Apparently, the demo's executable will work with the game's data apart from the DLC.
EDIT:
So the game launched today i believe.
Steam download is a whooping 75gb... for the 4k version i hear its 155GB..
Good Lord.... looks like SSD's soon wont be advisable for a PC gaming system, and we'll need to go back to at least 8TB spinners.
And for anyone wondering how you actually get the High Resolution Textures, you go to the game in your Library. In the large pane on the right, there should be a DLC table that lists the textures. Check the "Installed" check box, and it will start downloading the textures. Just keep in mind that the textures are a hefty 63GB download, which is almost as big as the rest of the game (~73GB).
Oh, and I keep the game on my SSD. I use a 1TB Crucial m.2 SSD. It's significantly cheaper than Samsung's offerings, but it also runs hotter... a lot hotter. In fact, without proper ventilation or heatsinks, it will thermal throttle during a lot of transfers. Its max temperature is 70C and it never gets over 63C (90% of tjMAX). My case (Thermaltake Core X9) wasn't exactly built for airflow, so I've had problems with the drive when installing large games. At one point, I just set a 120mm fan on the HDD drive cage and pointed it at the m.2 drives (they're both near the bottom on my ASRock board).