These three verses refer to the Three Nephites, and not to all of the twelve. They had requested the opportunity to remain on earth, to continue to work to preach the gospel, until Christ should return. They could not do that as mortals. Thus, there was a process of change: “for it did seem unto them like a transfiguration of them, that they were changed from this body of flesh into an immortal state.”
One wonders how they had the strength to continue in their process of preaching among mortal humanity after they had beheld “the things of God.”