Is it permissible for someone who began prayer as a follower (ma'mum) to become the leader (imam) later in the same prayer?
Chapter on Imamate and Congregational Prayer
Al-Mughni
Book of Prayer
Primary text
It is permissible only in the instance where the original Imam suffers an impurity (hadath) and appoints a successor to complete the prayer for the congregation. This case has already been detailed.
Supporting text
If two individuals join the Imam for part of the prayer, and after the Imam concludes, one leads the other in completing the remainder, there are two opinions regarding its validity. If both individuals simultaneously intend to be the Imam for the other, or the follower for the other, their prayers are invalidated due to prior stated reasons.