For sleep as a metaphor of death see on John 11:11. Paul calls attention to the fact that there are some who will not die, but who will be translated from the imperfect physical state to the perfect heavenly state. This instantaneous change will make them like the resurrected saints (see GC 322, 323; SR 411, 412).
All be changed.
This “all” includes both those who are alive when Jesus comes and those who have died. The former instantaneously exchange their mortal bodies for immortal bodies; the latter are raised with immortal bodies (cf. on 2 Cor. 5:1-4).