flipping AI Cap religion

when you try to eliminate an AI religion and use GP to change to your own, the old religion does come back. How high does the religious pressure need to be before the old religion is gone. I've seen it vary from +12 to +16. I do flip religion in surrounding cities if I havent razed them

If is holy city it will always generate pressure. Only way to remove holy city status is capturing that city and using inqvisitor

thanks, i thought a Great Prophet would have the same effect removing the religion permanently