Changing one's intention for Salah from an obligatory Salah to a supererogatory one

Q 3: I pronounced Takbirat-ul-Ihram (saying: "Allahu Akbar [Allah is the Greatest]" upon starting Prayer) and then started the Salah (Prayer) with the intention of it being an obligatory Salah. I then remembered that I had not yet performed the two Sunnah (supererogatory) Rak‘ahs (units of Prayer). Is it permissible for me to change my intention from obligatory to supererogatory Salah without ending the Salah I am performing? May Allah bless you and grant you all goodness.

A: It is permissible for a Muslim who started to perform an obligatory Salah to change it to supererogatory Salah for a sound reason. For example, if someone started to perform Salah alone and then a group of people came and he wanted to perform Salah in congregation with them or for another similar valid reason, such as (Part No. 5; Page No. 298) the questioner mentioned, concerning changing the intention from obligatory Salah to Sunnah Qabliyyah (supererogatory Prayer performed before an obligatory Prayer on a regular basis), provided that there is enough time to offer the obligatory Salah in its due time.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.