Is it permissible to change a specific obligatory prayer into a specific supererogatory prayer with a determined number of rak'ahs, such as changing the intention of the Maghrib prayer of three rak'ahs into a supererogatory prayer of three rak'ahs?

Whoever enters into an obligatory prayer must complete it according to its original intention. It is disliked to change its intention to a supererogatory prayer without a valid reason, and it may be forbidden. It is permissible to change the intention for a valid reason, such as to catch the congregational prayer.

Regarding supererogatory prayers, it is best for them to be two rak'ahs by two rak'ahs, based on the hadith "The prayer of the night and day is two by two." However, it is permissible to pray them as four continuous rak'ahs or three continuous rak'ahs, as was established by the action of Mu'adh and the Prophet's (peace be upon him) approval. An example is the Fear Prayer (Salat al-Khawf) where the Prophet (peace be upon him) would pray four rak'ahs while his companions prayed two by two, and in the Maghrib prayer, six rak'ahs while his companions prayed three by three. This indicates the permissibility of the first prayer being obligatory and the second supererogatory.

It is permissible to change the intention in a supererogatory prayer by increasing or decreasing for a valid need, provided that it does not violate the legal characteristics of the prayer, such as praying five or seven rak'ahs, except in Witr prayer. The best is to pray supererogatory prayers two rak'ahs by two rak'ahs, unless there is a specific reason for otherwise.