It's just random. I was able to kill Pyzjin several times and she always dropped items from other nameds between 12 and 15. That's just my experience.
Where does it say random loot will change? All I saw was that shared loot tables are expansion specific (i.e. you won't get kunark loot off velious mobs)
Okay that was my bad, misunderstanding what they meant. For some reason my brain translated "within the same expansion" to actually meaning "the expansion you are currently in will drop random loot from that expansion". What they actually mean is that every expansion will have its loot dropping every which way within itself.
We are all guessing, but conventional wisdom is that these mobs will still drop the epic items and no other mobs will. There's a difference between "rare" and "named" in the code. Most epic part droppers are named but not rare.
From what I have seen, it swaps the whole loot-table. We picked off Rahotep (L16) in NRo and looted what would otherwise be on the Thaumaturgist (L18) from Befallen. Blackened Wand Thaumaturgist's Robe Dagger of Marnek Bronze Spear Update: Picked off Rahotep again... Looted: T-robe & Dagger of Marnek (again) Hollwed Iron Mask Next kill: Thunderhoof quiver Permafrost crystals Wooly Spider Silk Net