Is it valid for one person to follow another in prayer if the original Imam steps away and a new Imam is appointed from within the remaining group?
Chapter on Praying with Impurity and Other Matters
Al-Mughni
Book of Prayer
Primary text
If two individuals catch part of the prayer with the main Imam, and upon the Imam's completion, one follows his companion while the other intends to lead them, this action is valid. This is because it is considered analogous to the permissibility of appointing a successor (Istikhlaf).
Supporting text
Those who do not permit appointing a successor (Istikhlaf) would not permit this subsequent action (one person following another after the main Imam finishes).