They provided new subclasses (indeed, this is where the term "subclass" got started in D&D.) These subclasses could be played right alongside the original ones. Those with altered roles were especially ripe for such a thing. Certainly, you could mix and match to your heart's content: a Slayer Fighter next to an original Paladin in the front, with an Arcanist (aka original) Wizard next to an Elementalist Sorcerer in the back. Or what-have-you.
No. Because from everything I've seen, it is intended that the "One D&D" way is intended to truly replace the old. You are not supposed to use unmodified backgrounds, for example, because backgrounds now provide key parts of your starting resources (a feat, for instance.) Sure, you can convert them, but you still need to convert them. Likewise races, or "species," will require conversion. Not a ton of it, but some. It is significantly more like 3.5 vs 3.0: the two games have mostly the same mathematical structure, so you can use adventures from "5.0" in "5.5" as you like, and most options can be converted pretty easily or swapped out cleanly (e.g. Ranger will now be getting Expertise, backgrounds will require some adaptation), but the player-facing resources are not meant to be mixed together exactly as they are. With 4e, that was always 100% the intent and results of Essentials: alternate takes that were fully inter-compatible with existing player content.
That someone makes the argument does not say much. People make a lot of arguments... especially when it comes to edition warring. That said, it's not an argument I've ever seen made, and would certainly muck up the clean alternation between "N.0 = brand new game, N.5 = just a minor update."
Beyond that: I don't see the changes to things like Skill Challenges, or even Stealth, to be indicative of an edition change, not even a half edition. The thing that pissed people off about 3.5, the thing that made it a "revised edition," was that it was intended to replace, not complement. Content that wasn't given the rubber stamp was implicitly suspect (and that was honestly a good thing, 3.0 actually IS worse for balance than 3.5, if you can believe it.) Players were expected to adopt the new, tweaked rules and not mix the two together, though effort was made to make it so prior adventures could still be run with minimal changes.
That's always been my standard for a "N.5e" change. Does it ask players to use replacements for the mechanics they've been using, e.g. races and classes and spells, rather than offering alternatives to those things? If you are replacing the core player experience, even if that replacement is just a bit of conversion, it's an N.5e. If players and DMs can keep using exactly what they've been using and add in stuff from the new material or use new material and add in old stuff at their leisure with little to no conversion, then it's still the same game, it just got a big supplement.
Hence, even though it's a ton of stuff, Incarnum+Bo9S+all the extra base classes like Hexblade and Binder do not constitute a "half-edition" or "revised edition." Because they are just opt-in new options for the same game. Skills & Powers, from everything I've heard, is also not a revised edition, it's just a ton more options. The original Unearthed Arcana is not a revised edition. Etc.
By this metric, I could in fact actually see an argument that 2e was effectively a "revised edition" as I use the term: it preserved much of the continuity, but did in fact make changes large and small that meant you probably shouldn't mix and match player-facing materials together. It wasn't a major departure like the differences between 1e, 3e, 4e, or 5e, but it was still a shift and players really were expected to learn a modified set of rules, even if much of it would be very familiar.
For 4e, RC, MM3, and MV were useful expansions of existing material. Errata had been official rules updates from day 1, so a book officially collecting errata up to that point was simply a convenience. No class fundamentally gained or lost any mechanics. The recommended math for skill challenges changed, but the actual process of doing one remained pretty much identical; it had less effect on player experience than the 1e->2e shift from attack matrices to THAC0. Likewise, there is nothing really wrong with using unmodified MM1/2 monsters in any 4e game. They're just likely to be not quite as exciting, and to take longer to defeat; the "updated" math is simply there to make the monsters offer faster, riskier combat. I have personally had DMs who mixed the two together freely without any adaptation at all.
Did 4e change across its run? Yes and no. Stealth changed, skill challenges changed a bit, all classes got errata over time (mostly just preventing obviously problematic stuff, like infinite damage if you could consistently hit an enemy with a certain Ranger power.) That is change. But it's not change any different from getting supplements and patching up little problems, something every edition and half-edition has done. So those changes, while certainly change, aren't on the same level. There were a lot of VERY upset people in the wake of 3.5e's launch, accusing WotC of fleecing their customers by forcing everyone to buy new books after only a couple of years. That criticism never occurred with 4e and Essentials, despite all the misinformation people spread (and continue to spread) about 4e, because Essentials didn't do that.
Hence why I care about this distinction. "Half-edition," "revised," whatever we want to call it, there is a meaningful gap that occurs here. And we can see this logic in a perfect test case: Pathfinder. It is not, despite what Paizo claimed, 1:1 compatible with 3.5e. Many classes work differently. Many classes got new features or significant reworks. And what did people call it? "3.75e," the analogy being "if 3.5e was only half an edition, this is the same process happening a second time, so we add half of a half, bringing it to 3.75." If you want to play PF1e, you do need to do some conversion. It isn't a ton of conversion, because the underlying math remains the same (which is why PF2e exists), but the classes and their features are different and need to be relearned.
It is useful to draw a distinction between on the one hand the kinds of slow, iterative, piecemeal change that literally all editions (and pretty much all descendants) of D&D go through; and on the other hand, the sharp, conversion-inducing, feature-rewriting, fundamental-rule-tweaking changes that "revised" editions specifically bring to the table. Why enforce weird new vocabulary or jargon when we have a perfectly useful term already applied in exactly this way, the "half-edition" concept? Why dilute a useful term by making it mean "any time anything's added"?
Now, if someone wants to argue that Essentials represents something similar to a half-edition, well sure, make that argument. Recognize where the similarities break down. Potentially, propose your own terms, e.g. "Essentials, and other things like it such as Unearthed Arcana, Skills and Powers, or Bo9S, are a smaller step than a revision, but a bigger step than most supplements, united by their large scope and often tinkering with how players get into the game. I say we call these 'N.1e': it recognizes that a meaningful change has occurred, but one that remains pretty much the same game with more stuff." That's a perfectly cromulent position and proposes a potentially useful new bit of terminology. We could say, for instance, that Pathfinder's "Ultimate" effort was PF1.1e, not an edition revision, but providing alternate takes on existing classes. Likewise, one might argue that the 13 True Ways supplement for 13th Age is "13A 1.1," since it added a ton of major new options without contradicting anything that came before.
And this can be applied to 5e as well. Many folks talk about "2014 5e," which is pretty clearly synonymous with "5.0e" in the above taxonomy. Things have changed and grown over the past almost-nine years, and many folks recognized a shift in structure and approach around the time of Tasha's. (Really it started earlier but Tasha's was when it became obvious to most folks.) We can think of Tasha's as being 5.1e: it's still the same game, but we're getting new, alternate options. Like the different types of dragonborn, which do not replace the original, just providing alternate options. (Though you absolutely should use gem/metallic/chromatic, because PHB dragonborn mechanically suck.) Or the shift from short-rest abilities to Prof-per-long-rest abilities. It's not enough to make any kind of real break, you can totally still mix and match stuff from "5.0e" and "5.1e," but it's definitely some kind of change. "One D&D" is very clearly shaping up to be very, very similar to 3.5e: classes will be reworked, spells will be changed, some common subsystems will require at least a little conversion (e.g. backgrounds.) Consider, for example, the proposed changes to Magical Secrets; under the new rules, how could one even begin to make sense of picking lists from a single class when there is no such thing in "One D&D"? Instead, you'd have to either just use the new Bard, or do some conversion work, or just...not actually use the "One D&D" classes at all, which defeats the whole purpose of trying to use the new material. It doesn't all play nice together, but it can be adjusted to do so with a little effort. Hence, "One D&D" in its current playtest form looks very much like a "5.5e": a revision of 5e, that keeps the same overall rules structure while making pervasive updates that players must learn to use and must perform a few conversions to adapt to.
