From what I understand, they can't replace him at all until he's dead. That's how Dies Horribly found out Chief is dead; the viper clan tried to make him chief and it would have worked if he had accepted it. See this page and the following. Let's just not talk about how dumb a plan it is to perform chiefing ceremonies on random goblins on the off chance that their chief might have died recently enough that they didn't get replaced yet.