Only a kind of difference of degree - I edited the post to be a little clearer about that.
If someone says this GH game isn't really GH and that this isn't what they signed up for, they'd be pretty right about that. If you said this was essentially GH just with some little changes, you'd also be right about that. It's about what's within the GH "genre," and what's in or out is largely subjective - about personal tastes and feelings, not about objective % of things changed.
To some folks, a genre swap is a minor thing - the nouns are what counts (I ran a Call of Cthulu-inspired FR game, and it was recognizably FR, at least until everyone started turning into fish-people...).
But, I would assume that when you pitched the game to the players, you told them the campaign was Call of Cthulhu inspired no? There was likely no bait and switch - they knew from the get go what they were getting into.
Now, was it a Forgotten Realms campaign or not? It was "recognizably FR", so, I'm presuming that most of the canon stuff was easily recognizable. So, would it be fair to say that this was a Forgotten Realms campaign? While specific Mythos creatures might not exist in FR, there's certainly more than enough "mysterious creatures from beyond" in the setting that it shouldn't cause too many ripples.
Or going back to the Thule example. Note, the restriction is not no cantrip casters in the setting. It's "no cantrip caster PC's". So, at that point at least there are no canon changes at all. There most certainly are wizards and clerics in the setting. It's just that they're all NPC's. Now, I have deviated from setting canon by allowing paladins and monks, that's true. But, is that really enough to say that it's no longer a Thule campaign? Seriously? The inclusion of a class or two is enough to make a setting "not that setting" anymore?
All these lore threads have just made me really, really glad that I don't run published settings very often. I would have zero interest in ever having these conversations at an actual game table. Far better to stick with home-brew settings where I don't have to deal with this crap. Sorry, I know the players at the table far better than some writer plugging away for 10 cents a word. I am far more interested in the table having a good time than satisfying some bizarre notion of setting fidelity which appears to be a moving target at best.
Good grief, we've got [MENTION=23751]Maxperson[/MENTION] talking about how it would be fine to have Wizards of High Sorcery in Greyhawk, but only if we change the name.

Yeah, because THAT'S the key point. The idea that I'd run or play in a 100% kosher canon setting makes me want to stick a pen in my eye. What happened to creativity and making the game your own.
Going back to Greyhawk for a second, are the Paizohawk AP's Greyhawk or not? They are supposed to be. But, are they canon or not? Is the fact that I added Grippli to my Savage Tide AP campaign supposed to mean that my game was no longer truly a "Greyhawk game"? Gimme a break. If running canon means that I have to limit myself to the ideas that some other guy had, count me out.