Steam store page says "You already own Divinity: Original Sin"
And if it's like the early access game I know so far, it will be seamless... how would a key revoke even WORK with steam? Never heard of something like that. Developers can't just "unlist" games in your library that are listed as "owned" and listed on the steam page as public purchase option.
So seriously, stop worrying about that
It will be seamless and painless like it ALWAYS is (probably)
What I am wondering is how we get the backer rewards.. because the steam key alone won't give us em, they must be extra keys for DLC?
That said, if it turns out I am wrong then this would be the first time I ever saw a dev delist a key that shows you the ACTUAL game as owned in steam. But hey, ye never know.
Maybe I am totally wrong ;p