A somewhat formal/literary expression meaning 'to become' or 'to be (in a certain state)'; used to indicate a change into a state or an outcome, often in written or formal contexts.
来月から制度が新しいものとなる。
From next month the system will become a new one.
本件は正式に承認されたため、規則となる。
Since this matter was officially approved, it will become a rule.
彼は次の学期にクラス代表となる予定だ。
He is expected to become the class representative next term.