Is it permissible to change a vow of distributing 15 bags of rice (5 kg each) to one bag weighing 20 kg of rice?

A vow of devotion and obedience must be fulfilled in the manner in which it was vowed by its maker, for the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him." If you are unable to fulfill it completely, and the vow was temporal, with a deadline, and you achieved part of what was vowed by the deadline, then fulfill what you were able to and offer expiation for the remainder, which is an expiation for breaking an oath. However, if the vow was absolute, without a time limit, and you achieved part of it, then give in charity what you were able to, and the remainder remains a debt upon you, which you should give in charity whenever you are able.