Is it permissible to convert churches into mosques in lands conquered by force, and how is this reconciled with the opposition to converting mosques into churches in Andalusia and what Mehmet the Conqueror did with the Hagia Sophia?

One must beware of disputing with the people of falsehood unless one is capable of refuting their arguments. Cities that Muslims conquered by force, after they had been established by disbelievers, are not permitted to have new churches or places of sale built within them. As for what existed before the conquest, there are two opinions: the permissibility of retaining them or the obligation of demolishing them. Ibn al-Qayyim holds that the Imam should do what is most beneficial for the Muslims. If the benefit lies in taking possession and removal, he should do so; if it lies in leaving them, he should affirm their existence. Building mosques is an establishment of truth, while demolishing them and converting them into churches is an injustice. Cities established by Muslims are not permitted to have new churches built in them, and if any exist, they should be demolished. Conquered lands taken by force are not permitted to have new churches built in them, and scholars have differed concerning what existed before the conquest; what is most beneficial for the Muslims is to be considered. Cities conquered by treaty (peace) follow the terms of the treaty. If the Muslims made a treaty with them that the land belongs to them, then they should not be prevented from building whatever they wish. However, if the treaty was that the land belongs to the Muslims, then the matter follows what was agreed upon in the treaty, and it is better to adhere to the conditions of Umar ibn al-Khattab, may Allah be pleased with him.