For most of D&D's history, magic hasn't had a source, it just is.
No disagreement with your comments about Krynn and Mystara, but I don't agree with what I've quoted.
Here is the relevant passage (AD&D DMG, p 40):
All magic and cleric spells are similar in that the word sounds, when combined into whatever patterns are applicable, are charged with energy from the Positive or Negative Material Plane. When uttered, these sounds cause the release of this energy, which in turn triggers a set reaction. The release of the energy contained in these words is what causes the spell to be forgotten or the writing to disappear from the surface upon which it is written.
The triggering action draws power from some plane of the multiverse. . . . [T]here is a flow of energy - first from the spell caster, then from some plane to the area magicked or enspelled by the caster. The energy flow is not from the caster per se, it is from the utterance of the sounds, each of which is charged with energy which is loosed when the proper formula and/or ritual is completed with their utterance. This power then taps the desired plane (whether or not the spell user has any idea of what or where it is) to cause the spell to function. . . .
When spell energy is released, it usually flows to the Prime Material from the Positive or Negative Material Plane. . . .
Release of word/sound-stored energy is not particularly debilitating to the spell caster, as he or she has gathered this energy over a course of time prior to the loosing of the power. It comes from outside the spell caster, not from his or her own vital essence. The power to activate even a first level spell would leave a spell caster weak and shaking if it were drawn from his or her personal energy, and a third level spell would most certainly totally drain the caster’s body of life!
Because spells tap power from other planes, any improper casting is likely to cause the spell not to function . . . or to malfunction . . . Such happenings are covered in the various chances for spell malfunction. If your players inquire as to how spells work, or fail to do so, you can explain, without difficulty, the precepts of the AD&D magic spell systems.
Magic absolutely had a source in AD&D!
The Weave is just the Forgotten Realms name for the magical interface that allow you to use the magic in the multiverse. There are no special rules for it. The only thing that is official is that there is an interface between the raw ambient magic that fills everything (or if you prefer to take a more scientific angle, just say that this "raw magic" is mass-energy), and the spellcaster's will.
<snip>
So no real change. Clarification or codification, but it doesn't invalidate prior lore or force any specifics.
The Weave is very specific, and quite different from the AD&D lore that I just quoted. From the Basic PDF (p 81):
The worlds within the D&D multiverse are magical places. All existence is suffused with magical power, and potential energy lies untapped in every rock, stream, and living creature, and even in the air itself. Raw magic is the stuff of creation, the mute and mindless will of existence, permeating every bit of matter and present in every manifestation of energy throughout the multiverse.
Mortals can’t directly shape this raw magic. Instead, they make use of a fabric of magic, a kind of interface between the will of a spellcaster and the stuff of raw magic. . . .By any name, without the Weave, raw magic is locked away and inaccessible; the most powerful archmage can’t light a candle with magic in an area where the Weave has been torn. But surrounded by the Weave, a spellcaster can shape lightning to blast foes, transport hundreds of miles in the blink of an eye, or even reverse death itself.
All magic depends on the Weave, though different kinds of magic access it in a variety of ways. . . .
Whenever a magic effect is created, the threads of the Weave intertwine, twist, and fold to make the effect possible. When characters use divination spells such as detect magic or identify, they glimpse the Weave. A spell such as dispel magic smooths the Weave. Spells such as antimagic field rearrange the Weave so that magic flows around, rather than through, the area affected by the spell. And in places where the Weave is damaged or torn, magic works in unpredictable ways - or not at all.
That's quite different from what Gygax wrote in his DMG. Just for starters, it locates magic in the world itself ("the stuff of creation") rather than in other planes; and the caster manipulates magic (via the weave) rather than channeling energy from another plane. That's real change. It's certainly not
clarification of what Gygax wrote.