The answer is in your first statement. The believers who will be on this earth in the flesh at the time of His arrival, are the dead believers that will be changed into their spiritual body. Scripture clearly states when Christ return, those in the flesh will be instantly changed in the twinkling of an eye. Blink your eye and that's how fast it will happen. All flesh will be changed, however not all souls will be saved. Flesh and blood can not enter heaven, so naturally, all flesh is changed when Christ return. What will they be changed from? People living on earth in the flesh will be changed from a dying flesh body into a living spiritual body. If this confuses you, then read Luke 9:27-36 for a deeper understanding of what our spiritual bodies will look like. In Luke 9:27-36, Christ allowed us to see that people who die, don't have a soul floating out there in space, nor are their souls lying in the ground, or in a holding place. Verse 33 states that Moses and Elijah are in their spiritual body. In other words, they are not in a holding place. Notice, the bible referred to Moses and Elijah as "men" in spiritual bodies; and Peter, James, John recognized Moses and Elijah without an introduction from Christ. Pay attention to the description of the bodies because ours will look the same as Moses and Elijah when we die.
Luke 9:27-36 (Amplified Bible)
27However I tell you truly, there are some of those standing here who will not taste death before they see the kingdom of God.
28Now about eight days after these teachings, Jesus took with Him Peter and John and James and went up on the mountain to pray.
29And as He was praying, the appearance of His countenance became altered (different), and His raiment became dazzling white [[a]flashing with the brilliance of lightning].
30And behold, two men were conversing with Him--Moses and Elijah,
31Who appeared in splendor and majesty and brightnessand were speaking of His exit [from life], which He was about to bring to realization at Jerusalem.