Is it permissible to change one's surname to the middle name "Sultan" instead of the father's first name "Karim" or the family name "Mirali"?

No one is permitted to attribute themselves to other than their father. If a person's family is associated with the Ismaili sect, they are not obligated to mention it in their official documents; it suffices for them to use the name of their father and grandfather. A person must attribute themselves to the father who begot them, and the husband has no right to refuse this. If the husband insists on it, there is no harm in attributing oneself to the grandfather, because the grandfather is also a father, and attributing oneself to the grandfather is in reality like attributing oneself to the father. If a person does not mention their father's name and only uses their grandfather's name, they must mention their family name to preserve their lineage.