Kjos, the game is essentially the same, just with a developer commentary and achievements. There's also supposed to be an alternate ending, but I haven't been able to access it. I got a free upgrade, as I have the GOG version. I essentially rushed through it just to see what the new ending was like, only to find that it was exactly the same as I'd remembered it. I wonder if all the achievements need to be reached first. Or maybe there's something that needs to be done earlier. In any case, I'm pretty sure it is there, as I took a look at the Steam achievements which match the GOG achievements. When all are unlocked, there is an "alternate ending" achievement, so it must be possible to access it somehow.