List of changes to the dungeons are listed here, discovered by a player.  The AC changes are definitely valid.  If you would like confirm the changes by replying to this post.

AC
Spider Queen now shoots poison fields in melee range.
P2: Champion Graveling Breeder can no longer be damaged by the spike traps. It has to be damaged directly.
CM
P2: Players can no longer damage mobs through walls. (mobs moved further from walls so players can't reach)
TA
UP: Malrona's projectiles can no longer be reflected/blocked. Untraited warden will still absorb it. (I assume the same goes for the spiders in Arah P1 and P3 but haven't tested yet)
SE
P1: Can't pull Tazza separately from Kaeyi anymore. (previous patch) However the old tactic will still work. As soon as Tazza teleports a distance away from the stairs, Kaeyi will deaggro back to his spawn point while Tazza stays in the big room.
P3: General Molradovich's Breath of Flame attack (fire rings) can no longer be removed with a condition cleanse skill.
P3: Clown Car now faces opposite direction, so the mobs spawn closer to you. This change changes nothing for PUG tactics; if the group party takes a step back in the PUG spot to break LOS, the mobs will run up the ramp as usual.
P3: The 2 Dredge Champs after the Clown Car are no longer stationary.
CoE
Mark T-B34RC3 (Turret Golem boss) is no longer stationary.
Arah
P3: Magecrusher is no longer stationary. (previous patch)
FotM
Molten Furnace: During the Weapon Test section you can't stack in the corner to avoid fire AoE anymore. (previous patch)
Molten Boss: Shockwave skipper achievement is fixed and rewarded after completion of the fractal molten duo. (If you dodge all waves)

http://www.reddit.com/r/Guildwars2/comments/2780ff/unlisted_dungeon_changes