Why must the body be changed?
The body must be changed because this corruptible, mortal body cannot enter glory or stand in God's presence. It must put on incorruption and immortality, being made fit for eternity. Living believers, like the dead raised, cannot go to glory in the old body; they must receive the new body.
Les teaches that the reason for this change is that we cannot go to glory in this old body. Those who die and experience resurrection will have a new body, but living believers must also be changed. Paul says, "We shall not all sleep, but we shall all be changed," in a moment, in the twinkling of an eye, at the last trump. The dead will be raised incorruptible, and believers will be changed suddenly. This corruptible must put on incorruption, and this mortal, this body prone to die, must put on immortality. We have to be made fit for eternity in God's presence and be given this new body.