To be honest there haven't been that many (atleast not in WDW).
The main ones that come to mind ar rethe deterioration of sound and show quality, but that's for another discussion and I'm sure is nto what you're looking for
.
Lets go through room by room, shall we?
Entrance and Facade - The Hearse was added in the 90's and the Pet Cemetary was added in recent years (not sure exactly). The Madame leota Tombstone was added back in 2003. There have ofcourse been additions of safety signage etc, but nothing terribly noteable.
Foyer - The master Gracey portrait has remained unchanged since the parks opening (although recently someone mentioned the turning around of the slides, but that was probably someone being an idiot. Someone else mentioned the cobwebs being removed from the chandelier, but I don't know if this has merit (I hope not, it has stood out in my memories since I was little).
Stretching Rooms - Again, nothing but deterioration.
Load - The bat Stanchions have been refurbished and replaced over the years, and an annoying and seemingly unnecessary safety spiel has been added to the Doombuggies. On several occasions I have had the CM lower the safety bar right before the Ghost Host's "I will lower it for you" spiel. Slightly off Topic, but maybe it means the lowering mechanism doesn't work in some buggies. That or the Ghost Host is getting Lazy:slurp: .
Staircase Room - The lights of the Candleabra seem to have dimmed to the point where you won't notice it unless you look for it. Irk.
Portrait Hall - No changes here. Maybe paintings have switched places, but I'm just speculating. I don't have evidence to support this. mark it off as no changes.
Library - The "Donald Face" chair has been removed and replaced with your garden variety old mansion armchair. Irk again.
Music Room - The pianist's shadow effect I think has been slightly dumbed down, as I seem to remember one point where the arms were slightly more synched and jerky. When a note would hit the arm would appear to slam down on the keys and quicky raise back up a second later. Now it seems to have more of a waddling effect where the shadow tilts back and forth byno means in synch with the music. Irk Irk.
Staircase - There's always the rumor of there being a screaming human in one of the spiders webs (the large one on the right) that was removed early on, but I don't personally believe it. Maybe I'm mistaken. I don't know. Choose to note it or not. Lets move on.
Short Hallway right before Suit of Armor and Endless hall- I think that sometime in the nineties a billowing curtain in front of a "window" was added, as I don't seem to remember it from my childhood (and trust me, I have very distinct memories of the mansion from my childhood, and not because I loved it!). Who knows, maybe it was always there, I just don't think it was.
Endless hall - Again, the "Donald" Armchair has been removed. Irk Irk Irk. Remember what I said before about the Candleabra dimming to the point where you have to be looking for it? Irk Irk Irk Irk.
Conservatory - Is it just me or was there a time when the Doombuggies didn't go backwards through the conservatory? I seem to recall they turned back around after turning towards the endless hall. I was suprised when this happened as I was riding the mansion several years ago. Can anyone corfirm?
Corridor of Doors - See above for mention of Doombuggies going foreward. other than that I believe the Corridor is largely unchanged.
Seance Circle - You may be right about Leota's hair being Blacklit. I don;t know why they would change this, but whatever. I don't think that the Glowing Green light was always there, but maybe I'm wrong. I don't believe WDW's Leota Projector has been converted to Digital yet, but please correct me if I'm wrong. Other than that no changes.
Ballroom - Largely unchanged except for the Hidden Mickey of plates which was not originally there.
Attic - Ofcourse, probably the most famous change of WDW's mansion, the Bride. Once a dark mysterious face, now a glowing green ethereal one. Negative Change in my mind. The pop up ghosts have been dressed up (or down, however you look at it) slightly to look wedding worthy, but our attic did not get the full makeover as Disneylands, and the Audio remains screams and heartbeats as opposed to DL's Minor-key version of the famous Wedding March. Rumor has it that the attic in DL's Haunted Mansion (currently down for Rehab) will be welcoming back the Hatbox ghost when ti comes back up. lets hope this happens at Disneyland and will later happen at Disney World.
Graveyard - No major changes.
Tomb - No major Changes except maybe replacement of the Hitchikers behind the Mirrors (I think they were resculpted at some point, but I have no proof). Little Leota also remains the same.
Exit - No changes.
So there you have it. Unlike DL's haunted mansion, our HM has changed very little since opening day. There may be more changes than what I mentioned, there may be less, but atleast now you have the gist of it.
I hope this helps!
Yensid "I have a thing for the Mansion" tlaw1969
) and it looked like his hads have been changed. 
