IOTL, Plato's Academy at Athens was forcibly closed down in the 6th century by Justinian, as Athens was still a hotbed of paganism. Now, a pagan academy surviving to the present day would be ASB- the later Byzantines wouldn't allow it. So WI, instead, the Academy is Christianised, at least on the surface, and survives- the Byzantines have no incentive to get rid of it, and the Ottomans are unlikely to.
Possible effects:
Athens remains a sizable city, although as a university town not a political or trade centre.
Greece now contains the world's oldest university- more than 1000 years older than Al-Karaouine in Morocco, 1500 years older than Bologna or Oxford, and 700 years older than the University of Nanjing. What effect does this have on the War of Independence or Philhellenism?