If a person is in purgatory, that person has already bent his or her will to the desires of God. So, no, purgatory does not change a person’s will. God will never change a person’s will. He proposes the Truth for our belief. ‘The Act of Faith’, ‘Act of Hope’, and ‘Act of Love’ prayers all end with the words: In this (faith-hope-love) I intend to live and to die. These are reflections of the will of the inner person.
Now Saint Paul talks about the struggle against sin. God allows us this struggle for our benefit. While our desire to follow God and submit to Him may be very real (we submit our will to God), we must strive to overcome bad habits, temptations and vices, and our concupiscence. Virtue is acquired by practice - it does not happen all at once when we change our minds and obey God. (Sometimes we don’t even know at first what God wants.) He also allows the struggle so that we pay penance for our transgressions - to satisfy the demands of justice. True, it is an imperfect justice because we can never repay what we owe almighty God, but it works for our benefit and God accepts our efforts. Here is Saint Paul: 14For we know that the Law is spiritual, but I am of flesh, sold into bondage to sin. 15For what I am doing, I do not understand; for I am not practicing what I would like to do, but I am doing the very thing I hate. 16But if I do the very thing I do not want to do, I agree with the Law, confessing that the Law is good. 17So now, no longer am I the one doing it, but sin which dwells in me. 18For I know that nothing good dwells in me, that is, in my flesh; for the willing is present in me, but the doing of the good is not. 19For the good that I want, I do not do, but I practice the very evil that I do not want. 20But if I am doing the very thing I do not want, I am no longer the one doing it, but sin which dwells in me. Romans Chapter 8
As you can see, Saint Paul himself struggled with committing sins, but that did not negate his will to obey God. I would argue that not every sin is proximately caused by intentional will although many are. And even if they are so caused, we can immediately seek forgiveness and make amends (beware of the sin of presumption, which I am not addressing here). Some sins (caused by past or remote intentions) were decided upon long ago and now proceed out of bad habit or weakness. (You all can beat me up for this one if you want. I will wait for it.) Un-confessed, un-repented venial sin weakens the soul and weakens the will. God in his mercy has allowed us to enter purgatory when He knows we love Him but in our weakness we cannot completely follow Him with the perfection He desires and deserves. There is more to it, but that is a start.