You're right, modernising can be a good on dark rides. But I think With The Haunted House, on a technical level outside of its ride system, it was never that modern, even in 1992. Bubbleworks had loads more animations and better animatronics, and that opened 2 years earlier. I think the reason it isn't isn't modern is because John and Keith wanted the ride to be as theatrical as possible and didn't want it to be ultra realistic.
If you think of it as well, in your case, why is Hex regarded as the best madhouse in the world when loads others are way more modern and technically superior? Even the Haunting has more animatronics and very smartly hidden projections, so why is Hex regarded as the best? Because of its setting, ambience, and Music! Hex has no animatronics, but the way it's executed is what makes it amazing. You don't need the latest tech to create a great attraction. I think this was or is the same case with The Haunted House.
Sure they could add more with the same design and style like the original ride, but I still firmly believe if they implement a certain amount of Modern Tech, it could ruin the experience unintentionally, especially with projections and screens. The best thing they could do if they want to replace a scene is to keep the design faithful to the 1992 original for the new scene, but don't overly modernise it, outside of safety standards and reliability of course.
Many of the best regarded Ghost Trains still use mostly very simple animations such as props on animatronic sticks, simple illusions, and great music and lighting, etc, The Haunted Mansion, Terror Castle, The Orignal ghost Train at Blackpool, and Kneobles. Kneobles Ghost Train is still updated annually to this day ever since it opened in the 70s, but it doesn't go overboard with the modern tech.
If alton wanted a new Ghost train from the ground up, with the facade destroyed, the area rebuilt from the ground, a Modern Ghost Train would be very valid and I agree with you on that aspect, but with this parks 30 year old Ride, I don't think it would work as well as many people would think.
Here's a few examples on how to keep a classic Ghost Train still great after many years: