What is the ruling on the aforementioned vow that was associated with committing a sin? Is it permissible to amend it by increasing the hardship? Is it permissible to revert to its original form? What are the implications of doubting it? And is expiation for the vow sufficient to atone for the sin?

A vow of obstinacy and anger is like an oath; the one who made the vow has the choice between fulfilling it or offering expiation. Fulfillment is not obligatory. Doubt about a vow does not make it binding, because the default state is the absolution of one's responsibility. It is not permissible to change a vow after the condition upon which it was contingent has occurred. However, there is no harm in increasing seeking forgiveness, and we hope for the forgiveness of sins through seeking forgiveness and righteous deeds.