"Kahn" is a pretty common Indian surname. While, there might be few Hitlers left in the wold, I don't think everybdy named Kahn (or Green, for that matter) would change their names in reaction to the actions of one man.
I suspect there would have been quite an Augment witch hunt after the Eugenics Wars. And judging by reactions to Augments in Enterprise and Deep Space Nine, it would seem that Khan blood would be an instant ticket to persona non grata
status in the Federation even 300 years later. Not something any sane person would wear on their sleeve.