What is the ruling on changing the intention of a fard prayer to a nafl prayer after hearing the iqamah for another congregational prayer, in order to catch the virtue of the second congregation?

There is no harm in changing the intention of a fard (obligatory) prayer to a nafl (supererogatory) prayer in order to catch another congregation. This is because the worshipper is considered to be praying alone as long as they have not caught the first congregation. Jurists have stipulated the permissibility of this, provided there is sufficient time to perform the obligatory prayer.