It likely feels more like a sequel if you choose the "played it before" option at the beginning of the game-- that introduces the new content much earlier. If you tell it that you haven't played before, it takes much longer before it starts introducing the new content.
The original was interesting in its kind of a game, but more of a experience. The narration was excellent and there were some interesting nooks to find. I definitely didn't find everything. (I think there was an award to not play the game for a year).
There's about the same amount of old and new content.
> The script for this new content is longer than the script for the entire original game.
If you don't want more content you don't have to buy the remake. The original works fine.
It's not an upgrade, it's a remake. They recreated all of the original content from the ground up
After looking it up, they actually didn't bother re-recording the voice lines from the original so now half the narration is by someone a decade older.