The Ruling on Changing the Intention of Prayer from an Obligatory to a Voluntary One, and Vice Versa

Question:
He says: Shaykh, may Allah preserve you, sometimes I pray and while I am in the prayer, what is the ruling on my changing my intention? And do the regular voluntary prayers also fall under this?

Answer:
Changing the intention from an obligatory prayer to a voluntary prayer is permissible if there is sufficient time remaining. However, changing the intention from a voluntary prayer to an obligatory prayer is not valid, and it invalidates the prayer; the voluntary prayer is invalidated, and the obligatory prayer is not achieved either. Likewise, if one shifts his intention from one obligatory prayer to another, this is not permissible, and both prayers are invalidated. Yes.