Sim, a razão do retcon é que os Klingons nos seriados do TOS estavam sofrendo de um vírus criado artificialmente para melhorar suas habilidades físicas.
Por Memória Alfa
The Klingon augment virus was a modified form of Levodian flu that threatened to wipe out the Klingon race in the mid-22nd century. It was inadvertently created by Klingon researchers who were attempting to bio-engineer enhanced warriors using DNA from genetically-modified Human embryos left over from Earth's Eugenics Wars.
na época da nova série ST: TNG, uma cura evidentemente foi encontrada e administrada universalmente.
A cure to restore Klingons to their proper appearance was presumably found some time between 2268 and 2293, as Kang was seen as a smooth-headed Klingon in TOS: "Day of the Dove" and as a normal Klingon in VOY: "Flashback". The first time normal Klingons were seen on screen was in Star Trek: The Motion Picture set in the early-2270s.