Is it permissible to change the intention of a supererogatory fast to a make-up fast after having fasted several days of supererogatory fasts, knowing that I was unaware of the obligation of making up fasts before observing the supererogatory ones, and is it permissible to change the intention during the same day?

It is not permissible to change the intention of a voluntary fast that has been completed so that it becomes a compensatory (qada') fast, because a compensatory fast requires the intention to be made the night before, based on the hadith: "Whoever does not make the intention to fast before dawn, there is no fast for him." This is the opinion of the majority of scholars. Similarly, it is not permissible to change the intention after completing an act of worship, and an absolute fast does not suffice for a specified fast. It is permissible to observe a supererogatory fast for someone who has compensatory fasts from Ramadan due, as long as there is sufficient time to make up the fasts before the next Ramadan. The six days of Shawwal fast are an exception to this.