Verses 55-58 indicate that this transformation happens progressively as the Christian stops sinning.
To briefly address the difficulties herein it can be seen through study that the Greek word translated "twinkling" can just as accurately mean "to cast".
It can also be seen that the word translated "in a moment" can mean "undivided".
With this understanding it then becomes clear that the "undivided casting of the eye" or uninterupped attention directed towards (just as Peter walked on the water as long as His attention was fixed upon Jesus, Matt 14:25-33) the sound of the trumpet will bring about this transformation.
The trumpet is of course the message or voice of Christ, as can be seen from Revelation 1:10.
It then can become clear that an undivided "casting of the eye" "or gaze" at Jesus (the sound of the trumpet) will bring about this transformation from corruptible to incorruptible.
This is the process of a Christian being conformed into the "image" of incorruption or the image of Christ Jesus.
In 1 Thessalonians 4:16 we see the same sort of trumpet blown, however this time the sound of the trumpet (more specifically the intervention of the Lord) is used to wake the sleeping or sinful brethren as the Rapture parable illustrates.
After the willfully sinful "sleeping" brother or sister is awakened (stops the willful sin) through the Lord's intervention or "discipline" (the time of discipline is referred to as the "Day of the Lord"), the Christian's personal transformation from corruptible to incorruptible can resume.
After the disciple referred to as the "Day of the Lord" the Christian who has stopped willfully sinning then joins the brethren who remained faithful to Christ (and sucessfully resisted willfully sinning) in the air (where the devil is the prince of the power of the air). Joining the brethren who have remained faithful to Christ "in the "air" indicates that this process which can be referred to as "Correction for discipline" happens in this present life on earth.