Sheikh Hamed Al-`Ali, instructor of Islamic Heritage at the Faculty of Education, Kuwait and Imam of Dahiat As-Sabahiyya Mosque, states that: If someone makes nadhr (a vow) to do an act of obedience to Allah, he or she must fulfill it. But if a Muslim makes a nadhr to do anything of the lawful things such as studying a certain subject then he opts to change his field of study, then what he is supposed to do is to offer expiation for breaking the nadhr the same way he does for breaking an oath. This is because making nadhr to do what is mubah (permissible) is subject to the same juristic ruling that applies to taking an oath.
This is based on the hadtih of the Prophet (peace and blessings be upon him) that reads: “The expiation of (breaking) a vow is similar to the expiation of (breaking) an oath.”
the aforementioned hadith is reported by Muslim and is only confined to making a vow to do the mubaah. The Prophet (peace and blessings be upon him) is also reported to have said: “Whoever vows to do an act of obedience to Allah must fulfil it.”