Is it permissible to change one's intention during prayer, such as someone who intended to pray Witr as one rak'ah then prayed two rak'ahs, so is it permissible for him to intend it as three while he is praying?

If a Muslim intends to perform Witr with one rak'ah, it is not permissible for him to change his intention thereafter to perform Witr with three rak'ahs. If he prays Witr with the intention of a specific number of rak'ahs, it is not permissible to increase that number after commencing the prayer. The Witr prayer becomes invalidated by intentionally adding two rak'ahs. And if he intentionally adds even one rak'ah, his prayer is likewise invalidated.