They're changing the behind-the-scenes-BACKSTORY to it. They went about this really clever.
The original, Fuller designed Klingons were genetically bald (fits with his "completely-start-from-scratch" approach). But we only know this from behind-the-scenes comment. It never made it on screen, thus it was never "canon" in the first place.
The only remnants of this approach is that all klingons in S1 have always been bald - even tough the new explanation was it was because of "ritualistic shving for war" - the klingons were already shaved before the war, in alternate timelines, and in flashbacks with T'Kuvma as kids. And there was never a single exception, even when it would have made sense (low level Kronos street live, politicians, other houses with other traditions...)
But since they never actually made it canon - they can change that - which they did - by simply suddenly showing haired klingons, without actually tecnically retconning itself. The only thing that remains is the weird "conincidences" of S1. And in that "soft" approach, it makes a lot of sense to not immediately and entirely throwing the old look out, but have the old & new show up side-by-side to aknowledge the canonicity of both of them.