What is the ruling on changing the intention during prayer?

Question : If one intends the Zuhr prayer and then changes it to 'Asr, both are invalid, because he cut off the intention for Zuhr, and 'Asr is not valid because he did not intend it at the start of the prayer. However, if he changes it to a voluntary prayer for a reason, such as starting alone and then a congregation gathers, so he makes it voluntary to pray his obligatory prayer in congregation, it is valid, because the intention for a voluntary prayer includes the intention for an obligatory prayer. If he does that without a purpose, it is disliked, and the change is valid as we mentioned. It is also possible that it is not valid, as we mentioned regarding Zuhr and 'Asr.

Answer : No, it should be said: "It is possible that it is not valid," not "it is valid," for this reason: it is not permissible to exit an obligatory prayer except for a reason, and this man exited the obligatory prayer to a voluntary one. This issue requires establishing a criterion. We say: if one moves from a specific prayer to a specific prayer, neither is valid. If one moves from a specific prayer to a general one, which includes the specific one, it is valid. If one moves from a specific prayer, or from a general one to a specific one, it is not valid.
Clear. Yes, good. Moving from Zuhr to 'Asr is not valid; neither Zuhr nor 'Asr is valid. As for Zuhr, because he invalidated it, and as for 'Asr, because he did not intend it from the beginning. Moving from a general voluntary prayer to Witr is not valid, because it is moving from a general to a specific. If a person is praying Tahajjud at night, praying a general prayer, and then makes it Witr, we say it is not valid, because it is moving from a general to a specific. Good. Moving from the Zuhr prayer to a general voluntary prayer is valid. Why?
Because the Zuhr prayer actually contains two intentions: the intention of prayer and that it is Zuhr. He invalidated it being Zuhr, so the intention of prayer remained. Good. Moving from the Zuhr prayer to the regular voluntary prayer (Rawatib) of Zuhr is not valid. Why? Because it is moving from a specific to a specific. So Zuhr invalidated it, and the Rawatib must be Rawatib from the beginning. It is not possible for a prayer to be general at the beginning and Rawatib at the end. It is not possible. Yes.