1. Islam does not change the religious places of non Muslims. Thousands of religious places of non Muslims all over the Muslim world remained unchanged and unchangeable.
2. If the people of the area opt willingly to become Muslims and use the place as Masjid for them, they have that right. The people of Istanbul have willingly become Muslims and decided to use that place as Masjid for them. The place was used by people of the area as a Masjid for centuries till a ruler ( Mustafa Kamal) issued an order to stop Muslims from using the Masjid and announced it as a museum.
3. The Turkish court examined that order of Mustafa Kamal and found it illegal.
4. When a place is made a Masjid, it remains Masjid till the Day of Judgement.
No one is allowed to change a Masjid.