That required one retconn, Chekov's age.
Why would that have been "required"? Chekov being younger (although born earlier) than in TOS doesn't really serve any specific purpose other
than allowing McCoy to doubt his abilities until he proves his youthful genius. Chekov being the same age as in TOS would work out just fine in those
terms, since the storyline perfectly well accommodates undergraduates in Kirk's posse: a thirteen-year-old genius first-year cadet would fit right in.
That is, making nu-Chekov a graduate at seventeen after it has been spelled out that most people spend four years or more at Starfleet Academy requires us to believe that he enrolled at thirteen. (Or at something like sixteen, if he was even more driven and clever than nu-Kirk, but still...) It sort of undermines the one thing that is special about the career path of the main hero.