With the addition of minarets, Hagia Sophia was converted from a Christian basilica to an Islamic mosque. Candace Richards, Author provided

This bold inclusion of old material in a new monument for Rome led to a whole new recycling trend in architecture. Decorative elements such as columns, capitals and architraves were given new life in buildings of the fourth century CE. The trend became so popular that new laws were created to protect public buildings from being stripped of their decoration. Only if a building could not be restored was it permitted to recycle that building’s materials.

Source: Reinventing heritage buildings isn’t new at all – the ancients did it too