What change does God effect after a person changes what is within themselves, taking into consideration that human change leads to an automatic change in their condition?

Nothing on Earth happens automatically without Allah Almighty's decree. Whether it is a change or something else, the meaning of His saying, "Indeed, Allah will not change the condition of a people until they change what is in themselves" is that Allah does not alter the blessings and comfortable life a people possess unless they change what is within themselves from faith to disbelief, or from obedience to disobedience, or if they show ingratitude for the blessings, in which case Allah takes them away. Similarly, if people change what is within themselves from disobedience to obedience, Allah will change their state from misery to goodness and joy. This does not happen automatically but by Allah Almighty's decree and by taking the predestined means.