1 Thessalonians 5:23
KJV 1Thess. 5:23And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ.
NKJV 1Thess. 5:23 Now may the God of peace Himself sanctify you completely; and may your whole spirit, soul, and body be preserved blameless at the coming of our Lord Jesus Christ.
Looks pretty serious. Why change it? What was the matter with the KJV? And, if you really just wanted to use a more modern world, how about "until"?
It is this subject that concerns me most. This appears to hit right at the gospel of grace.
en
A primary preposition denoting (fixed) position (in place, time or state), and (by implication) instrumentality (medially or constructively), that is, a relation of rest (intermediate between G1519 and G1537); “in”, at, (up-) on, by, etc.: - about, after, against, + almost, X altogether, among, X as, at, before, between, (here-) by (+ all means), for (. . . sake of), + give self wholly to, (here-) in (-to, -wardly), X mightily, (because) of, (up-) on, [open-] ly, X outwardly, one, X quickly, X shortly, [speedi-] ly, X that, X there (-in, -on), through (-out), (un-) to(-ward), under, when, where (-with), while, with (-in). Often used in compounds, with substantially the same import; rarely with verbs of motion, and then not to indicate direction, except (elliptically) by a separate (and different) prep.
Okay, now that we see what it can mean in Greek, and understanding that Translating is not as easy as thought

if we consider a small preposition such as en...
Lets see what the SDA Bible Commentary says on this one.
Unto. Or, “at,” that is, at the time of the coming.
Nichol, Francis D.: The Seventh-day Adventist Bible Commentary : The Holy Bible With Exegetical and Expository Comment. Washington, D.C. : Review and Herald Publishing Association, 1978 (Commentary Reference Series), S. 1 Th 5:24