You have to make your own canon. Gene Roddenberry once stated that Klingons ALWAYS had ridges, they just could not depict it with the makeup available in the 1960s. That was good enough for me, and watching TOS I always imagined the ridges there. Then Trials and Tribble-ations
comes along and challenges my personal canon. I simply chose to ingore Worf's comments about the lack of ridges, as I choose to ignore Enterprise's augment virus explanation.
In my head the TOS uniforms are identical to the ST09 uniforms and Alternate Kirk is physically identical to a young Prime Kirk... there's just two different depictions of them. You have to keep in mind that you are not watching the events, you're watching a depiction of the events.
The various Star Trek series and films do not contradict each other as much as some nitpickers seem to think, and when it does it's best just to ignore the little things like forehead ridges or Tuvok's rank pips, and go with your gut feeling on big things like the Eugenics Wars/WWIII.