Ever tried interpretting blood glucose values between a Chilean man with type 2 diabetes and a broken hip, who speaks Spanish and German, and a Royal Navy doctor who speaks only English and doesn't want anything to di with diabetes, when you speak English, French and Spanish and had the forethought to bring a blood glucose units conversion table? Admittedly, we weren't discussing HbA1c, but it's an example of why internationally recognised untis are worthwhile!
There will be a period of running both units together. People newly diagnosed won't have to learn the old system at all.