On many occasions in Israel one person will lead pseukei d'zimrah but then step down to allow someone else to lead prayers; presumably because they simply don't want to continue. But often, because there may have been a delay in starting, someone will agree to 'get the service going' but then step down before Barechu.
I have davened with a particular chassidic group where the Rebbe was renowned for a long amidah. The Rebbe led maariv at the start, but someone else continued as shliach tzibbur after the personal amidah (i.e. said kaddish, then aleinu, etc.) because the Rebbe takes much longer than everyone else.
Question: Is it better to keep one shliach tzibbur for the whole of the service, or is it ok to change in the middle? If its ok to change, then at what point can this be done and for what reasons?