The first New Earth book explicitly has the transition between the two styles.
I see. Well, it's not the only one that purports to depict that. Howard Weinstein's The Better Man
alludes to McCoy still trying to get used to a new uniform design. (Although that novel is set in 2273 and was written at a time when the Chronology
put TMP in 2271, and before VGR: "Q2" canonically put the end of the 5YM in 2270, requiring TMP to be in '72-3. Since the novels assume TMP to be in '73, I choose to put The Better Man
very shortly after TMP -- right between Ex Machina
and The Darkness Drops Again
, in fact -- and assume it's those uniforms that McCoy is still adjusting to.) I think there are a couple of others as well.