Would you say it would piss people off who are huge fans of the original?
Possibly. I wasn't pissed off. I mean, seeing the remake didn't take anything away from how much I enjoy the original. What angers me a bit is when I see people reviewing the remake and saying that it is on par with (or possibly better than) the original. The people who say this are, from everything I can see, people who seem to think that the entire story banks on its ending. And while the ending of the Korean original is definitely the kicker, the rest of the movie is brilliant in various ways: How it's shot, edited, sound, character development, etc.
For me, an example of how a crazy twist ending doesn't make a bad movie instantly amazing (aside from this Oldboy remake) would be Book of Eli. Some people enjoyed it. Fine. I thought it was a sheer waste of time. I didn't care about the characters and it was shot like a flashy music video. So, when the reveal came along near the end, guess what? I didn't suddenly proclaim it to be an amazing film. Conversely, a movie that pops into my head where the twist ending was completely predictable yet the movie was such a good, enjoyable ride that I didn't care that I wasn't surprised is Shutter Island.
The point is, those who say that Oldboy (2013) is anywhere near Oldboy (2003) don't understand what makes the original so good. I recommend anyone who hasn't seen the original to see it before the remake (assuming you see the remake at all.) While seeing the remake won't completely ruin the original (how could it?) it will clue you in on the nature of the ending. By the way, I will totally spoil the ending of the remake if anyone wants. I wouldn't mind having a discussion on the failure of Oldboy (2013)'s twist in comparison to the original. I actually saw a video review of the remake by a guy who claimed that he saw the original just a month before seeing the remake, and he thought that this ruined the remake for him and encouraged people to see the remake prior to the original! Let's hope this guy isn't procreating.