Is it permissible to change one's intention for prayer from shortening (qasr) to completing (itmam) or vice-versa, if the traveler realizes the nature of the Imam's prayer (shortened or complete) after joining him in prayer?

If a traveler enters a prayer that can be shortened (whether as an Imam, a congregant, or praying alone), there are two scenarios for him:

The First Scenario: If he intended to pray a full prayer (non-shortened) at the takbirat al-ihram (opening takbir), or if he left his intention open, or if he hesitated between shortening and completing the prayer, or if he doubted his intention, then in all these cases, he must complete the prayer fully. If he shortens it, his prayer becomes invalid.

The Second Scenario: If he intended to shorten the prayer at the takbirat al-ihram, he has two options: First: He may complete his prayer by shortening it, provided that he maintains the intention of shortening throughout the prayer and nothing occurs that necessitates him completing it fully. Second: He may complete his prayer fully, by changing his intention from shortening to completing, or by hesitating or doubting during the prayer, in which case he must complete it fully.

If a traveler prays behind a resident (one who is completing the prayer), he must complete the prayer fully. If he prays behind someone he knows or believes to be a traveler, he may shorten the prayer behind him. If he is unaware of the Imam's intention and prays behind him, connecting his intention to the Imam's intention, the soundest opinion is that this connection is valid.

It is not permissible to change the intention from completing the prayer to shortening it. The congregant must follow his Imam if the Imam completes the prayer.