That’s a great question and reveals a heart that truly wants to understand.  So, the portion in 1 Thess. 4:13-17 says, “But we do not want you to be uninformed, brethren, about those who are asleep, so that you will not grieve as do the rest who have no hope. For if we believe that Jesus died and rose again, even so God will bring with Him those who have fallen asleep in Jesus. For this we say to you by the word of the Lord, that we who are alive and remain until the coming of the Lord, will not precede those who have fallen asleep. For the Lord Himself will descend from heaven with a shout, with the voice of the archangel and with the trumpet of God, and the dead in Christ will rise first. Then we who are alive and remain will be caught up together with them in the clouds to meet the Lord in the air, and so we shall always be with the Lord.”  (NASB)

Those who are ‘asleep in Jesus’ are dead.  (Compare John 11:11-14.)  Those who have ‘fallen asleep in Jesus’ are in Heaven with Him.  2 Cor. 5:8 tells us, “We are of good courage, I say, and prefer rather to be absent from the body and to be at home with the Lord.”   When the Lord returns to ‘rapture’ His saints, He will bring all those believers who have died with Him.  (They have been with Him in Heaven without their bodies.)  These who have died in Christ, will retrieve their bodies (wherever they are, whatever condition they may be in), and they will rise to return to meet the Lord ‘in the air’.  AT THE SAME TIME, those who ‘alive and remain’ will rise to meet the Lord ‘in the air’ also.  They will be ‘caught up together with them’ so that all saints will meet the Lord in their changed and glorified bodies.

The ‘changing’ of the bodies is described in 1 Cor. 15:51-55, “Behold, I tell you a mystery; we will not all sleep, but we will all be changed, in a moment, in the twinkling of an eye, at the last trumpet; for the trumpet will sound, and the dead will be raised imperishable, and we will be changed. For this perishable must put on the imperishable, and this mortal must put on immortality. But when this perishable will have put on the imperishable, and this mortal will have put on immortality, then will come about the saying that is written, “Death is swallowed up in victory. O death, where is your victory? O death, where is your sting?””  This shows that not all believers will die, but all believers will be changed.  The perishable bodies will be changed to imperishable bodies.  The mortal bodies will become immortal and fit for existence in Heaven.  How fast does it take to make this change?  In the twinkling of an eye.  (I read once that this is about 1/60 of a second!)

When does this change take place?  At the last trumpet.

When does the Lord return to get us?  When the trumpet of God sounds.

They happen concurrently!

Why is the Lord so concerned about us having our original bodies changed?  Why not just give us new bodies?  1 Thess. 5:23 says, “Now may the God of peace Himself sanctify you entirely; and may your SPIRIT and SOUL and BODY be preserved complete, without blame at the coming of our Lord Jesus Christ.”  When the Lord saved us, He saved our complete person…spirit, soul, and body.  All are important to Him.

On a side note, notice how that Paul wrote in 1 Thess. 4:15, “that WE who are alive and remain until the coming of the Lord…”  Paul lived in anticipation of the Lord’s return in his lifetime.  Each day draws us closer to the day of Christ’s return.  Romans 13:11 says, “Do this, knowing the time, that it is already the hour for you to awaken from sleep; for now salvation is nearer to us than when we believed.”  The day of the Lord’s return (our salvation) is nearer now and grows nearer each day.  Maybe today!  (342.6)