"You haven't changed" can refer to someone's personality, as in, their personality has not changed after so many years. It can also mean clothes, as in, the wife of the former client is saying that the person has not changed their clothes and they're shocked. The joke is playing on the different interpretations of "you haven't changed". 
At a conference , I unexpectedly encountered the wife of a former client , whom I hadn ' t seen in 20 or more years . ' ' You haven ' t changed ! ' ' she exclaimed . ' ' There ' s plenty of time for that , ' ' I replied . ' ' The dinner isn ' t until this evening . ' '