Haunted Mansion runs 16 hours a day and has for decades. Now, I'm sure they do maintenance at times, but there's no way it's anywhere near the level they do for others parts of the park (e.g. Rise of the Resistance, which closes early). I would be shocked if they did any refurbishing at all when they switch it to Haunted Mansion Holiday.
The "it's old" argument does not hold water in even the vaguest sense of the word.