Exactly, which is why Walter Koenig didn't want to point out the mistake of having Chekov and Khan remember each other in
TWOK when the Chekov character hadn't yet been introduced at the time of "Space Seed." He realized how easy it would've been for them to just write Chekov out of those scenes and replace him with one of the other supporting characters. And Koenig didn't want to lose out on a substantial role, so he stayed quiet.
And heck, something similar happened in
Generations. The character of Demora Sulu was originally written to be
Chekov's daughter, but they changed it to Sulu's daughter when they got Koenig and James Doohan to sub for Leonard Nimoy and DeForest Kelley. It shows you just how arbitrary and interchangeable most of the supporting characters were.
Fixed it for you.