I mean... you have Death Knights in Warcraft 2 and Necrolytes in Warcraft 1 who were quite literally raising the dead. Death Knights specifically were slain Necrolyte souls affixed to dead knights and soldiers of the alliance. It was very well already part of the "lore" (which was barebones as well) back then.
Retcons? Not really. Lore expands overtime and first Warcraft titles had very little lore to begin with compared to huge ass lore of Warcraft universe we have now.
And don't worry, we will have plenty of spectacles there. I'm pretty sure we won't just stumble over Kael'thas there chilling in some castle and screeching "merely a setback" at us. There will be a story.