I think that the market rules where the games will go. If a different art style will support the sale, than the decision is a no-brainer in my opinion.
I also support the Idea of a "Art Remastered" Version - especially to not reduce the effort the old artist did. He/She might need the game for her reference list and you might need her work in future games. Therefore, two versions or a modus to switch art styles in game will be best in my opinion.
I would consider a small fee, but not buy the whole game because of new art, because I think the old art was already decent