Perhaps I'm missing some context, but what is a "sea change"?
And the translators of the KJV didn't replace anything with anything, they were translating the word apostasia in a way consistent with how the word is used.
What you seem to be ignoring is how "departing" can relate to the sense of leaving or abandonment. If I depart from the faith, if I depart from Christ, that's a departure, that's a departing--an apostasy. Which is consistent with how the word is used.
Instead you latch onto a single possible translation, "departing", and then bring all your assumptions and presuppositions with you, and then violently force them into the word, and then claim anyone and anything that doesn't agree with you is part of some great cover-up to hide the truth.
That, frankly, is ridiculous and no one should take what you are saying seriously.
Here is how St. Jerome's Vulgate translates 1 Thessalonians 2:3
"Ne quis vos seducat ullo modo: quoniam nisi venerit discessio primum, et revelatus fuerit homo peccati filius perditionis,"
Jerome translates apostasia as discessio, a withdrawal, conceptually identical with "departing" in the sense of leaving someone or something, of turning away, etc.
The Peshitta translates apostasia as maruwt, "rebellion", from the adjective maruwd "rebellious" "unruly".
ܠܡܳܐ ܐ݈ܢܳܫ ܢܰܛܥܶܝܟ݂ܽܘܢ ܒ݁ܚܰܕ݂ ܡܶܢ ܐܶܣܟ݁ܺܡܺܝܢ ܡܶܛܽܠ ܕ݁ܶܐܢ ܠܳܐ ܬ݁ܺܐܬ݂ܶܐ ܠܽܘܩܕ݂ܰܡ ܡܳܪܽܘܕ݂ܽܘܬ݂ܳܐ ܘܢܶܬ݂ܓ݁ܠܶܐ ܒ݁ܰܪܢܳܫܳܐ ܕ݁ܰܚܛܺܝܬ݂ܳܐ ܒ݁ܪܶܗ ܕ݁ܰܐܒ݂ܕ݁ܳܢܳܐ ܀
In the 18th century Russian translation of 2 Thessalonians 2:3 we see
Да никтоже вас прельстит ни по единому же образу: яко аще не приидет отступление прежде, и открыется человек беззакония, сын погибели,
The word is otstuplénije, and it means "retreat" or "digression", that is, to abandon one's position, to turn away from one's obligations, etc.
Seriously, just look at how apostasia is used/translated in various languages.
You are cherry-picking your favorite way to translate it, and ignoring the nuance and how the word is actually used, and how readers of the Bible have always understood its meaning--and how it has been translated, and the intent behind the word and the translation choices in Bible translations across time, languages, and cultures.
If there is some great conspiracy to hide "the truth", then it's been going on since Jesus' earthly ministry, and you're going to have to blame Jesus, the Apostles, and the entire Christian Church of every time and place.
-CryptoLutheran
Sea change: Sea change or sea-change is an English idiomatic expression that denotes a substantial change in perspective, especially one that affects a group or society at large, on a particular issue. It is similar in usage and meaning to a paradigm shift, and may be viewed as a change to a society or community's zeitgeist, with regard to a specific issue. Wikipedia
Zeitgeist: The spirit of the age; the taste, outlook, and spirit characteristic of a period. Wiktionary
This is the truth: Paul originally taught the rapture to his Thessalonian flock: 1 Th 4:16-17. They were experts on the subject with supporting verses like 1 Th 1:10 and 1 Th 5:1-3 (the very violent beginning to the Trib).
His flock had received a fraudulent letter, made to appear to be from Paul, stating that they had missed the rapture and were in the Trib. The whole purpose of 2 Th 2:1-3 was to correct his Thessalonian flock, after they had been deceived by a fraudulent letter. In that light, please read 2 Th 2:1-3 BEFORE KJV came along with a total misunderstanding of the purpose to 2 Th 2:1-3.
Please also note that the Liddell & Scott Greek-English Lexicon carries the two main definitions to apostasia: (1) defection; revolt, and (2) departure; disappearance. It is easy to see that definition #1 relates to a falling away. However, definition #2 clearly relates to the rapture, as departure, in Greek, most closely relates to disappearance, as in physical departure.
Here are 2 Th 2:1-3 from Bibles BEFORE KJV came along with their very misguided sea change in verse 3 by replacing departure or departing with falling away. Here is what 2 Th 2:1-3 really said before the KJV in 1611. Paul was correcting his flock about a fraudulent letter they received, telling them they had missed the rapture and were in the Trib (Day of the Lord). Therefore, in verse 1 below, Paul is strictly raising the topic of the rapture, to then point out the deception suffered by his flock (verse 2), then he corrects them (verse 3):
2 Th 2:1 (Geneva Bible): Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our assembling unto him,
2 Th 2:2 (NLT): Don’t be so easily shaken or alarmed by those who say that the day of the Lord has already begun. Don’t believe them, even if they claim to have had a spiritual vision, a revelation, or a letter supposedly from us.
2 Th 2:3 (Geneva Bible): Let no man deceive you by any means: for that day shall not come, except there come a departing first, and that that man of sin be disclosed, even the son of perdition.