If you are seriously dating a man now with the intention of changing him once you both get married, I would suggest you drop that idea today, because he is not going to change! It won't happen.
If you are not pleased with what you see in the man you are dating now and are not willing to live with him the way he is in the event of both of you getting married, then he really cannot be the right one for you.
Mary's case is an example of what happens when you marry a man with the foolish hope that you will change him when you both get married. She married Collins even though he had always beaten her up while they were dating and he was also a womaniser. He would beg for forgiveness and promise to be a better person if Mary would give him another chance, yet he even got worse.
In spite of what she was going through in the relationship, she went ahead and married Collins with the idea that she would change him, persuade him to follow her to church, stop him from beating her, stop him from womanising and so on. Of course her plans did not work! Collins never honoured their marriage and he never stopped beating her.