On certain occasions a chazan has to be replaced (see O Ch 53 (17)).
If the error occurred in one of the intermediate blessings, the replacement chazan starts from the beginning of the brocho in which the mistake was made (see Oruch Hashulchan 126 (4)).
(In the light of the related questions listed below) are there any requirements for the replacement chazan e.g. that he has not answered boruch hu uvoruch shemo to the repetition so far, or that he has paid attention to the repetition so far?
related questions:
Why don't some people answer Boruch Hu Uvoruch Shemo in Chazzan's repetition of Shemona Esrei?
Why do we still do chazarat hashatz?
(less related): Chazzan cannot start the repetition of the Amidah? Does anything change for his replacement?
