What is the ruling on changing the intention from one obligatory prayer to another during the prayer?

The questioner:
He says: Honorable Shaykh, may Allah grant you success. If a man joins the imam in the Maghrib prayer while he is forgetful and thinks it is the 'Isha' prayer, and in the second rak'ah he remembers that it is the Maghrib prayer, so he changes his intention to Maghrib, is his prayer valid? And what is the rule regarding changing the intention in an obligatory prayer?

The Shaykh:
It is not permissible to move from one obligatory prayer to another during the prayer. It is not permissible. For this reason, he said: If one moves from one obligatory prayer to another, it is invalid, meaning the one he moved from and the one he moved to are both invalid. And if he moves with the intention from one obligatory prayer to another, it is invalid.

The presenter:
And length.

The Shaykh:
It is permissible to change the intention from an obligatory prayer to a voluntary prayer, but not to a voluntary prayer that is less than it. As for moving from one obligatory prayer to another, no.

The presenter:
He says: Honorable Shaykh, may Allah grant you success.

The Shaykh:
So it is not permissible to change a voluntary prayer into an obligatory prayer, whereas it is permissible to change an obligatory prayer into a voluntary prayer during its performance.