Prayer: Changing Intention from an Already Performed Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 743
If a person starts to perform prayers with the intention of the ẓuhr prayer and while performing it realises that he has already performed the ẓuhr prayer, he cannot change his intention to the ʿaṣr prayer. Instead, he must break his prayer and perform the ʿaṣr prayer. The same applies to maghrib and ʿishāʾ prayers.