Is it permissible to change the intention from an obligatory prayer to a voluntary one, and vice versa?

Question:
Is it permissible for the one praying to change his intention in the prayer and convert it to another prayer, whether it is an obligatory or a voluntary prayer?

Answer:
This requires a detailed explanation:
If one enters the obligatory prayer, he may not convert it to another obligatory prayer, and he may not convert it if he enters the regular voluntary prayer attached to the midday prayer into something else from the obligatory prayers, such as the regular voluntary prayer attached to the dawn prayer or the like. However, once he enters the prayer with an intention, he should complete it.
Except if there is a benefit, such as if he enters the obligatory prayer alone, and then a congregation comes that missed the obligatory prayer, so he converts it to a voluntary prayer, says the closing salam, and prays the obligatory prayer with them to attain the virtue of the congregation. This is a change for the benefit of the prayer. Or if he enters the voluntary prayer thinking that they have already prayed, and then it becomes clear that they have not prayed, 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 can join the congregation.

The presenter: May Allah reward you with good.