Is it permissible for a solitary worshiper to change his intention during the prayer from a fard (obligatory) prayer to a sunnah (supererogatory) prayer?

The basic principle is that whoever enters a prayer with the intention of it being an obligatory (fard) prayer must complete it as such. However, it is permissible for him to change his intention to that of a supererogatory (nafl) prayer if there is a need. For example, if someone remembers that he did not pray the obligatory prayer that preceded the current one after he has already begun the current prayer, he may change his intention from an obligatory prayer to a supererogatory one. Then, he should pray his obligatory prayers in their proper sequence, out of fear of falling into what the Prophet, peace and blessings be upon him, forbade: praying an obligatory prayer twice.