*I WILL CHANGE HIM/HER.*
He/she will change in marriage.
You can't change anyone. People only change to become their true selves when they see the need to change.
If anybody changes because of you to become who you want them to be, instead of who they were created to be, just know that you have destroyed them while trying to save them because for the rest of their life they will do all they will ever do to please you.
They will keep seeking you to validate everything they do.
That's not a life you want for someone you love.
If anyone is trying to change you to become who they want rather than who God created you to be, just respect yourself and run for your life.
Don't change to fit into people's lives when you know you will or might lose your true self in the process.
Be you and wait for him or her who will love you for who you are and not who they want you to be.
Note
It's two different things when someone is trying to help you change to become who you are destined to be and when they are trying to change you to become who they desire you to be which goes against who you are meant to be.
Open your eyes and spot the difference
Cheers.
If anybody changes because of you to become who you want them to be.