Is it obligatory to have the intention of following the successor (khalīfah), and thank you very much?

It is recommended for the Imam, if something occurs that necessitates his delegating someone to take his place (such as breaking wudu or nosebleed), to appoint one of the congregants who is qualified to lead the prayer. Otherwise, they (the congregants) should choose someone to lead them. It is not a condition to renew the intention of following the successor when he steps forward, because he is taking the place of the Imam. The Mustakhlaf (with a fatha on the lam) is the one who is being followed, and the Mustakhlif (with a kasra on the lam) is the Imam who has departed.