Does prayer become invalid if one intended the second رکعه (unit of prayer) to be the first due to satanic whispers, then changed that intention and completed the prayer with a correct intention?

The intention for prayer must precede it and be a general intention. The prayer is not invalidated if the worshipper intends the second rak'ah to be the first due to forgetfulness or a whispering (waswasah). However, if this is done deliberately without an excuse, the prayer becomes void. The worshipper should not give in to doubts and whisperings.

When a worshipper doubts having performed a pillar, it is obligatory for them to strive to resolve it. There are two scenarios: 1. If one of the two possibilities is more probable, they act upon it and perform the prostration of forgetfulness (sujud al-sahw) after the salām. If it is more probable that they prostrated, they continue their prayer and perform the prostration of forgetfulness after the salām. If it is more probable that they did not prostrate, they must return and perform the prostration they doubted, then complete their prayer and perform the prostration of forgetfulness after the salām. 2. If no particular possibility is more probable, they should base their action on certainty (which is the lesser amount) and perform the prostration of forgetfulness before the salām.

A person who is not certain of the number of rak'ahs they have prayed should base their action on the lesser amount (certainty), complete their prayer, and perform the prostration of forgetfulness before the salām.

It is important to note that doubt is not considered in three cases: 1. If it is merely an illusion or a whispering (waswasah). 2. If the worshipper experiences frequent doubts. 3. If the doubt occurs after finishing the act of worship, unless the worshipper becomes certain of the matter.