Imagine Sherlock stopped taking drugs, but never dropped his 'Shezza' look. So he's usually dressed in ratty clothes, doesn't shower as often as he should, his hair is messy & oily, skin looks bad and he rarely shaves, so he usually looks like a homeless junkie. Obviously this means Donovan & co are even less pleased to see him help out on crime scenes and he has a harder time getting into labs, but he's a genius so somehow he manages to gain reputation nonetheless, and everything happens the same as in canon. (established relationship with John that makes everyone question John's sanity and tastes would be a bonus!)

One day, he gets a case that requires him to look like a rich successful man, so he puts on one of the suits Mycroft keeps buying for him, gets a haircut, shaves etc. Cue everyone being STUNNED and disbelieving :) (and maybe he sees how much John appreciates the change and decides to keep it?)