Is it permissible for one praying alone to change the intention from offering an obligatory prayer to offering a voluntary one?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

In principle, the individual is obliged to complete offering the prayer if he started it as an obligatory prayer. However, he may change his intention from offering an obligatory prayer to offering a voluntary one with a legitimate excuse. An example of this is an individual who started offering a prayer and then remembered that he did not offer the prayer that precedes it in order after he bowed. So he is to change his intention from offering an obligatory prayer into offering a voluntary one and then he is to offer his obligatory prayers in due order.

He has to change his intention from offering an obligatory prayer so as not to violate the prohibition of the Prophet, sallallaahu ‘alayhi wa sallam, who forbade to offer the same prayer twice on the same day.[Ahmad and Abu Daawood].

Allaah Knows best.