Child-theme of a child-theme

I want to make some changes to Church and I gather from the Codex that the way to do this is to make a child-theme. But Church is already a child-theme. Can I make a child-theme of a child-theme? And how would I go about it?

Otherwise, is it safe just to edit the Church child-theme in the hope that there won’t be future updates that break it?

Thanks!