Is it permissible to change the intention of praying the Sunnah (supererogatory) prayer before Asr to the Asr prayer when someone enters to pray behind me as a follower?

There is no objection to a person performing a fard (obligatory) prayer being led by a person performing a nafl (supererogatory) prayer. The questioner should complete his supererogatory prayer, and then pray behind the mentioned person. It is not permissible to change the intention from a supererogatory prayer to an obligatory prayer during the prayer.