Learn Farsi: آن کلیسا قدیمی است.

In English, we use the same word for old people and old objects. In Farsi, قدیمی is strictly used for inanimate objects like buildings, cars, or ideas. The word پیر is only used for living things like people or animals. Since a church is a building, we must use قدیمی.

How would a native speaker say this sentence in a casual conversation?

In everyday spoken Farsi, آن is almost always pronounced as اون. Furthermore, the verb است typically merges with the word before it. Since قدیمی ends in a vowel, it adds a y sound before the shortened verb, becoming قدیمیه. So, natively it sounds like: اون کلیسا قدیمیه.

If we changed the sentence to mean those churches, would the word آن change?

No, the word آن stays exactly the same when it is used directly before a noun. To say those churches, you simply make the noun plural and say آن کلیساها. You only use the plural form آن‌ها when the word stands completely alone as a pronoun.