I consider this Pavel Chekov to be a different individual, genetically, than the original. I assume the others still look the same in-universe despite what we see, just as Saavik or Tora Ziyal or any other recast character is supposed to; after all, Spock Prime recognized Kirk and Scotty on sight. But Abramsverse Chekov is not only four years older, he's also a Wesley Crusher-esque prodigy, whereas Chekov Prime never really seemed all that exceptional in the brains department. So I think of this Chekov as sort of a cross-temporal older brother who was given the same first name by his parents. Perhaps Andrei Chekov and his wife always intended to name their first son Pavel, but in this timeline they had a different son four years earlier.