He just changed his appearance; his true nature hasn't changed.
Tips
usage
Often derogatory in mainland China - implies superficial change while the essence stays the same. In Taiwan, it can be used neutrally to mean a genuine transformation.
memory
改 (change) + 头 (head) + 换 (swap) + 面 (face) = change the head, swap the face - a cosmetic makeover that doesn't go deeper.
