Prayer: Changing Intention During Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 718:
If someone inadvertently (sahwan) starts performing the ʿaṣr prayer before he has performed the ẓuhr prayer and realises this mistake in the middle of the prayer, he must change his intention to ẓuhr prayer, i.e. he must make the intention that whatever I have performed until now, and whatever I am performing right now, and whatever I will perform, is all part of the ẓuhr prayer. After completing the prayer, he must perform the ʿaṣr prayer.