Why do living believers need to be changed at the rapture?
Living believers must be changed because they cannot enter glory in this old, corruptible body. This mortal body must put on immortality, and this corruptible must put on incorruption. They must be made fit for eternity in God's presence and given a new body at the last trump.
Les teaches that this change is a secret or mystery revealed through Paul, not mentioned by Jesus or the Old Testament. Since believers cannot go to glory in the old body, the living must be changed just as the dead are raised. At the last trump, in a moment, in the twinkling of an eye, the corruptible puts on incorruption and the mortal puts on immortality. This makes them fit for eternity in God's presence and gives them a new body.