As it currently stands, it is planned that the DL Mansion will not run the traditional show at all in 2024.
The Holiday decor on the exterior will come down in January to clear the way for the construction in the queue area, but many of the interior HMH elements will remain in place while the work is ongoing. Aside from some of those pieces being refurbed on an as-needed basis. The exterior decor will go back up shortly before the ride reopens as the Holiday. I've heard that'll be in July, but I assume construction could change that - I wouldn't expect it any earlier, though.
The Haunted Mansion got a big, thorough, and MUCH needed refurb during DL's Pandemic closure. So the ride itself is much less in dire straights on that front. But the Holiday does take its toll on the facility, since they typically want to keep downtime to an absolute minimum. They also know that losing the classic Mansion for a full year will not be popular, so it isn't being done lightly.
I've heard that *something* is being added to the ride during the closure that will be a permanent part of the traditional show but will debut with the Holiday reopening - hoping to hear more about what soon. And before anyone asks, it's not the Bat, who should be coming back, perhaps even before the January closure. There's a funny story with him but I'm not sure I can share it yet.
All this to say, weird times, but hopefully there's good stuff on the horizon that'll be worth the wait.