Savage Species is AWESOME...and this is why

[Terraism]

But they do convert retroactively when you apply a Lich template (from MM1) or every other "Undead" templete.

So a Ghoul that becomes a sorcerer has mixed HD (D4's and D12's), while a sorcerer that becomes a Lich or Vampire has all their HD converted to D12's.

One more bit that makes absolutely no sense, but we just go with. [Sigh.]