You are very wrong in those words.
When one truly allows God in, all problems can be dealt with.
People can change and that change is much more possible through God. The past may not be made different, but the future can be.
I believe that God CAN transform anyone who truly asks Him to do so. The problem is, we humans cannot ever know when that transformation has taken place, so we are entitled to be wary. We can even forgive him, but it is foolish not to keep track of these people.
That isn't what your original question was. You asked if God could change someone. The person has to change. God doesn't do it for them. He expects people to put forth the effort.