A transition from disbelief to faith or the reverse is a change in a created, contingent human state. Such a transition does not inform God of something previously unknown.
God's knowledge is eternal and encompasses things as they occur, with their times and conditions. The change is in the state of the known person, not in God's attribute of knowledge.
Without this distinction, the changing history of people might be incorrectly projected onto the divine knower, as though God's knowledge itself had to acquire new information.