Are believers already being changed into Christ's image from glory to glory?
Yes. Scripture says believers have already been transfigured into the same image from glory to glory by the Spirit of the Lord. Though this change is inward and not yet fully visible outwardly, God sees it, and as believers behold Christ as in a mirror, the world sees His reflection.
Les explains that even when it is not shown outwardly, God sees the inward reality: believers have already been transfigured into the same image from glory to glory, even by the Spirit of the Lord. He contrasts Israel, which was depicted as veiled from the truth of the Law and could not see that it was fading away, with believers who with open face behold as in a glass the glory of the Lord. As they look at Christ like a mirror, the world sees His reflection. The Law could never do that, but believers are under grace.