Is it permissible to change the intention of the Fajr prayer from obligatory (fard) to supererogatory (sunnah) after one has begun it individually?

Whoever begins an obligatory prayer is not permitted to change their intention to another obligatory prayer or a specific supererogatory one. If they change their intention, the first prayer becomes invalid, and the second does not commence. So, if one changes their intention from the obligatory Fajr prayer to its Sunnah (supererogatory prayer), both prayers become invalid. Allah the Almighty says: "And do not invalidate your deeds."