Nonono, you misunderstand, sorry for the lack of clarity in my previous post. Let me attempt to clarify what I meant.
Death Knights:
1st Generation: Souls from Orc Warlocks placed into the corpses of Stormwind Knights. (Old Horde, Stormreaver/Twilight's Hammer Clan, Horde of Draenor.)
-Teron Gorefiend
-Ragnok
-Gaz Soulripper
2nd Generation: Jaded and bitter Paladins that Traveled North to confront the Source of the Plague/Men that were convinced to join the Cult of the Damned (The Scourge)
-Arthas Menethil
-Baron Rivendare
*The Warcraft III manual refers to Arthas as the first however, when referring to these they use plural versions of words like 'Paladin' implying there is more than Arthas. We also know that the next two Death Knights, Falric and Marwyn were 3rd Generation. Perhaps there were others that came shortly after Arthas, perhaps the Warcraft III manual was retconned.
3rd Generation: Paladins and Warriors slain fighting against the Scourge, that were reanimated and handed runeblades. (The Scourge)
-Falric
-Marwyn
-Thane Kor'thazz
-Lady Blameux
-Sir Zeliek
-Alexandros Mograine
-Dranosh Saurfang
-Instructor Razuvious
~Ebon Blade Split~ (Still 3rd Generation)
-Darion Mograine
-Koltira Deathweaver
-Thassarian
Basically what I had meant was, whether or not 2nd and 3rd generation Death Knights would really be considered different from one another. They arrive at the point of Death Knight by different means, however, the "Death Knight" that both generations become, is the exact same thing. Basically much different than say, either generation of Scourge Death Knights, compared to Gul'dan's Death Knights.
As I said though, even that is up in the air. Especially considering the limited information we have on the 2nd generation of Death Knights.