Is it necessary to change a newborn's name from Hamza to appease the father, after naming him Hamza in fulfillment of a vow, and is that considered fulfilling the vow and attaining Allah's reward?

You must be keen to please your parents unless it involves disobedience to Allah, for the Prophet, peace and blessings be upon him, said: "The pleasure of Allah is in the pleasure of the parent, and the displeasure of Allah is in the displeasure of the parent."

Therefore, you can name the child after your father to avoid his anger and distress. You can also retract the vow because it is a permissible vow that is not binding and requires no expiation according to the majority of scholars. However, to be on the safe side, it is better to offer expiation for breaking an oath.