For a change like Tan'nari into demons to change the thematic significance/meaning of a setting, that setting has to be heavily about demons. The core of 2e was not about demons, so we are not talking about any change of thematic significance. Now, your personal homebrew campaign might have been heavily about Tan'nari, but that's not really what we are talking about here.
It seems to be what at least some posters (eg [MENTION=2067]I'm A Banana[/MENTION]) are talking about.
Suppose that my game (which might be a 1st ed game, or an OD&D game - given that this thread has focused heavily, though not exclusively, on cross-edition changes of "canon") makes demons central, drawing upon their presentation in the MM and PHB as exemplars of chaos and evil who live in the Abyss and trade in the souls of mortals (per the Night Hag entry), and are liable to be "turned" by clerics whose presentation is essentially along the lines of the mediaeval orders of knighthood.
Were I to then adopt the new Planescape canon, which presents demons as, in effect, "aliens" from another dimension - and which to that extent therefore strips them of the religious/moral/thematic significance that they had in their prior incarnation and which I have made a key part of my game - that would be a major change to my game.
The contention of some posters is that the disruption posed by such changes is a good reason not to change canon. I disagree. If the Planescape stuff will be bad for my game, then I just ignore it - thereby leaving my game unchanged.
As I mentioned, it's not even necessarily a change. The prime plane is ignorant of a lot of what goes on. Nothing may have changed and they may have just been wrong the entire time.
Edit: Or even a retcon at all. Nothing is said prior to Planescape what the motivations of demons and devils out in the planes is. The addition of the Blood War is just that, an addition. It really changes nothing. On the prime plane demons are demons and devils are devils, just like the primes believe.
It's only not a change if you treat all the stuff that is implied but unsaid about demons - implied by their
name, their symbology, their relationship to clerics, the name of their home plane, the archetypes that they draw upon, etc - as irrelevant.
If you took all that stuff seriously - which, ingame, is the "clueless prime" lore concerning demons - then the Planescape changes are massive. The mere fact that the moral and religious beliefs of "clueless primer" is all retconned as false (or, at best, massively limited and misunderstood), is a massive change. It completely reconceives the moral and thematic "hub" around which everything revolves.
So you dont mind if Mindflayers are time traveling space creatures or spawn of the Far Realm or simply mutated humans as long as they all want to destroy the Sun? That is fair enough to me, everyone has their breaking point.
I don't understand. As far as what WotC (or TSR before them) publishes,
I don't have a breaking point. I just use what I like and ignore the rest. As well as the mind flayers' goal being to extinguish the sun - which I think is pretty cool - the DSG also had the "Alignment Wars" as an explanation for how drow, duergar etc got driven below ground. I think that idea is much weaker, and have never used it.
I also like the idea of mind flayers being from the future. And from the Far Realm. Ideally they'd be from both! (Though I've never had the need to think through how that would work and why it would matter - but in my current 4e game I have Ygorl being from the future and travelling backwards in time, as per The Plane Below, and that worked pretty well.)
Take your example of the Succubus. Obviously there was a designer in charge of the Demon/Devil section of the game who was just going crazy over this creature that was just blatantly filed under the wrong section and they were in the position to indulge their particular OCD. And good on them at least they were giving it a go, eh. And on the other hand it is just another good example of going too far and if you make enough of those small changes then by the end you end up with a God Asmodeus ruling Planet Hell while Demons are some kind of corrupted Elementals.
Again, if you don't like it don't use it. Who has a gun to your head forcing you to use all this "canon" you don't like?