What religions are accepted from a Person of the Book who converts away from their initial People of the Book religion, assuming we rule he is not affirmed in a non-People of the Book religion?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
If a Person of the Book converts to a religion other than that of the People of the Book, only Islam is accepted from him. This is the position held by Ahmad and chosen by Al-Khallal and his companion, and it is one opinion of Al-Shafi'i, because religions other than Islam are false creeds whose invalidity he has already acknowledged, thus he cannot be affirmed in them, like an apostate. However, a narration from Ahmad suggests that only Islam or the religion he was originally upon is accepted, because his first religion was previously affirmed for him, and he did not move to a better one, so he may revert to it if he returns. This is because he is moving from a religion whose adherents are affirmed to one whose adherents are not affirmed, so returning is accepted, like an apostate returning to Islam.
Supporting text
A third narration from Ahmad states that three things are acceptable from him: Islam, returning to his original religion, or a religion whose adherents are affirmed. This is based on the generality of the verse: 'until they pay the Jizyah out of hand while they are humbled' (Quran 9:29). If he converts to a religion of the People of the Book and we hold he is not affirmed therein, there are two narrations: one says only Islam is accepted, and the other says only Islam or his original religion is accepted.