I would think a remaster would almost be more work than a remake. From what I understand, the code on the original RDR was a mess, which is why they never remastered it for ps3/ps4. Plus they already have the built RDR2 world, they could just copy/paste it into a new game.Just hold on to your horses partner, there's a lot of hype surrounding this upcoming re-release of RDR1 because a lot of people want it to be a remake but nothing really confirms it is one.
Knowing Rockstar I bet it's a remaster, they've never remade a game and quite frankly they must be so busy with GTA VI that I'd be shocked if they actually devoted money, people and resources to rebuilding RDR1 in their updated engine.
But yeah, they would still have to add a ton of new coding for missions, locations, dialog, you name it. I'm hoping it'll be worth their time and they do it, but it is kind of unlikely with GTA VI on the horizon.