OK, you can put a uniform on Khan, it was done in "Space Seed", but the sideburns? How are those explained? He was given a uniform and a haircut?
If you're going to infiltrate Starfleet, the sideburns are kinda essential. I seem to recall Mr Norman, in "I, Mudd", carrying off a convincing infiltration of Kirk's crew.
And most shapechangers have no problem with sideburns.