Yes, the retcon reason is that the Klingons in the TOS serials were suffering from an artificially created virus intended to improve their physical abilities.

Per Memory Alpha

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.

by the time of the new ST: TNG series, a cure had evidently been found and universally administered.

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.

Yes, the retcon reason is that the Klingons in the TOS serials were suffering from an artificially created virus intended to improve their physical abilities.

Per Memory Alpha

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.

by the time of the new ST: TNG series, a cure had evidently been found and universally administered.

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.