There are three resurrections.
1 Cor 15:
20 But now Christ is risen from the dead, and has become the firstfruits of those who have fallen asleep.
21 For since by man came death, by Man also came the resurrection of the dead.
22 For as in Adam all die, even so in Christ all shall be made alive.
23 But each one in his own order: 1. Christ the firstfruits, 2. afterward those who are Christ's at His coming.
24 3. Then comes the end, when He delivers the kingdom to God the Father, when He puts an end to all rule and all authority and power.
The last enemy Christ defeats is death. Nobody alive is changed when Christ returns. This has been a huge misunderstanding throughout most of the church. There is a gathering of the remnant but nothing says they are changed at this time.
26 The last enemy that will be destroyed is death.
Christ reigns for 1,000 earth years. Then death is defeated and all those in heaven are resurrected and God brings them with Him when He establishes His new heaven and earth.
1 Thes 4:
14 For if we believe that Jesus died and rose again, even so God (NOT JESUS) will bring with Him those who sleep in Jesus.
You see here that it is God who brings those asleep in Jesus. Those who are alive will not precede (or prevent) those who are asleep in Christ because they are already with Him in spirit. To be absent from the body is to be present with the Lord. This is true in spirit from the moment of death.
As we see from the below Christ only comes back once and we see that it is appointed for men to die once. The only exception is for the living at the very end of the millennium. If Christ were to change all living believers when He first returns, then they would not be appointed to die once. This passage is given in context of the Lord's one and only return.
Heb 9:
27 And as it is appointed for men to die once, but after this the judgment,
28 so Christ was offered once to bear the sins of many. To those who eagerly wait for Him He will appear a second time, apart from sin, for salvation.
We learn from 1 Cor 15 that flesh cannot inherit the Kingdom of God. When does God establish His kingdom? At the very end, after the millennial reign of Christ. So the below is seen clearly in context to be AFTER the millennium and not before. Again, the LAST enemy Christ defeats is death. It isn't the first enemy He defeats.
1 Cor 15:
50 Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; nor does corruption inherit incorruption.
51 Behold, I tell you a mystery: We shall not all sleep, but we shall all be changed--
52 in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed.
53 For this corruptible must put on incorruption, and this mortal must put on immortality.
54 So when this corruptible has put on incorruption, and this mortal has put on immortality, then shall be brought to pass the saying that is written: "Death is swallowed up in victory."