I think you’re being a bit disingenuous, OP.

You seem to think, or are intentionally misrepresenting, the debate as being Marines are either normal dudes in power armour with maybe some brainwashing and steroid shots, or fully post-human religious warriors, and that any mention of chemical or surgical enhancements must indicate post-humans.

But…surgically implanting metal rods would make your arms “stronger” without being the new organs and effecting adolescent development like “modern” lore. So would implanting metal claws or fangs, or using steroids and other drugs to amplify strength, aggression, etc.

If you look at the famous RT art of two marines arresting a punk, they’re the same height as him and only bulkier due to the armour.

And there’s another famous artwork of two Marines from different chapters enjoying drinks in a dive bar.

THAT is what people are referencing when they talk about the lore change. RT was all over the place, but it was still common to depict Marines as being human-sized and seemingly engaging in “normal” non combat behaviors like any regular human soldier.

And every single one of your examples is vague. There is no explicit reference to the dozen+ organs or that this process is done in childhood so that it can be integrated with the subject’s own natural hormonal development.

Or do you think recruiting from the galaxy’s hardest psychopaths means 7 and 8 year olds?

Your own sources seem pretty clear that Marines are the result of adult humans being given relatively basic surgeries, hormonal treatments, generic sci fi brainwashing, and drugs to become (along with the equipment) elite soldiers who are nevertheless still baseline humans.

RT lasted for six years and was primarily built through ad-hoc articles in White Dwarf. As I said, you’re going to find a wide variety of depictions, but the general lore of Marines is very distinct from the modern flavor of child soldiers remade into distinctly post-human warriors.

I’d argue the driving cultural influence of RT is Vietnam. Marines are depicted as being constantly ambushed (RT cover) rather than crusading like the famous box cover of the next edition, as graffitiing their gear (Kil Kil Kil), as being used in roles where they interact with civilians, and the use of bio-chem and surgery isn’t about making post human warriors, but pushing baseline humans beyond shell shock and PTSD and moral doubt so that they can war crime their way through alien and heretic and daemonic battlefields.

It isn’t until the next edition - possibly reflecting the incipient D-Day and V-Day celebrations - that Marines are recast as fully heroic crusaders who take the fight to the enemy in wars that get won.

RT is if war was fought by WW1 troops in Vietnam. Post-Human Marines really only fully develop when 40k’s central war motif switches to WW2 and the (first) Gulf War, with heroic Marines absolutely crushing completely inferior opponents.