I'll give you ZA, they just updating most of the bosses in that instance with little lore reasoning.
However, in ZG both returning bosses came back in new bodies and their fights were completely different then before.
Nefarian and Onyxia came back because Deathwing resurrected his son, who turn Onyxia into an undead. Both (were in same fight) had completely different fights.
ragnaros and Geddon are elementals, they don't stay dead.
In Scholomance, Grandling I believe was explained in the questline in Battle for Andorhal and Barov is a spirit. Both have new fights.
For SM, I do not know the reasoning for Whitemane's returning, but I believe the fight is different.
So it seems, only ZA was rehashed, because the definition of rehasing is basically copy pasting.
---------- Post added 2013-02-20 at 05:10 PM ----------
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