Ruling on changing the intention during prayer in an obligatory or voluntary prayer

Is it permissible for the one praying to change his intention in the prayer and convert it to another prayer, whether that is in an obligatory prayer or a voluntary one?
Answer: This requires detail: If one enters an obligatory prayer, he is not allowed to convert it to another obligatory prayer, and he is not allowed to convert it if he enters the regular voluntary prayer of the midday prayer to something else, such as an obligatory prayer or the regular voluntary prayer of the dawn prayer or the like. Because whenever one enters a prayer with an intention, he must complete it, except if there is a benefit, such as if he enters an obligatory prayer alone, and then a congregation comes that missed the obligatory prayer; so he converts it to a voluntary prayer and says the closing salam, and prays the obligatory prayer with them to attain the virtue of the congregation; this is a change for a legal benefit.
Or if he enters a voluntary prayer thinking that they have prayed, and then it becomes clear that they have not prayed; so he converts it to a voluntary prayer and prays the obligatory prayer with them. If it is for this benefit, then converting it is required; so that he may attain the congregation. Yes.
The presenter: May Allah reward you with good.