Is it permissible to change a vow contingent on doing something that cannot be done, without expiation?

If a vow is an act of obedience to Allah, then it must be fulfilled in the manner in which it was vowed, based on the Almighty's saying: "And let them fulfill their vows," and the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him."

However, if you are unable to fulfill it due to an inability that is not expected to cease, then you must offer an expiation for a broken oath (kaffarat yamin), which is feeding ten poor people, or clothing them, or freeing a slave. If you are unable to do that, then you must fast for three days.