To transform something means to change that thing into something else. So in simple terms, we can say transform means change.
God can call anyone unto Himself because he loves us but God does not want us to remain the same. God wants us to change He wants us to be a better version of ourselves.
I understand that some people believe that you can come to God the way you are, that is true. But you cannot remain in God the way you are.
God wants you to grow to become a better person. God doesn’t want you to be stagnant.
In the book of Act 13:9, God used Saul but Saul had to change to Paul which is a better version of home Saul.
If you want to be called by God and remain useful in His kingdom, you need to understand some things.