On the SoA's changing, that has been shown for both Sora and Ventus already. For Ven, as Ruran pointed out, it were the inner symbols in the rings which house his friends' wayfinders in the later station while as a kid four years before BBS they only show a stylized fleur-de-lys and a six-pointed star in the middle.
Not to mention the outer symbols, which are fleur-de-lys in Ven's "kid"-station and Mark of Mastery-symbols in the "current" BBS-form and even Ven himself, as his "kid"-station lacks the shoulder-armor, chest-straps and his keyblade, all which he does have in the "current" platform.
The broken platform represented the rupture in his heart created by Xehanort, with the chunk missing at the side representing the piece that was ripped off to create Vanitas.
Sora's platform initially didn't even hold a picture of his own form as seen in "where the heart goes" when we see a four-year-old Sora's station while the inner rings house various objects connected to island life.
If we look at the other SoAs we know though, the background likely does represent something that has a huge impact on the heart in question, as Snow White has a flower field, Cinderella the palace steps, Aurora a maze of thorns and Belle even has a person, the Beast, at the background of her station.
IDK about Terra and Aqua (there's pasts are way too ambiguous) but if we want to go by the novels, Ven was orphan MX found in some nondescript world during his travels. I highly doubt he's from the KBG though, with Terra and Aqua I can see speculation that there may be civilizations at other local around LoD where they may have originated but the KBG is pretty apparently a waste land. Again, if we want to go by the novels (which are of questionable authenticity) then perhaps Ven saw the KBG as his "first true home" despite being from another world, much like how Kairi sees DI as her true home. Though I think it's more likely that the KBG is the background of his SoW because it just had such a huge impact on his life.
Ambigious or simply close to non-existent? Last time I checked we do not get anything about Terra's and Aqua's past except the fact that they were already training under Eraqus four years before BBS.
Ventus is a bit more murky, the fact that Xehanort found him on some nondescript world during his travels and decided to train him after seeing his potential is from the reports in BBS itself, so that can count as canon fact.
The orphan-bit though is from the novels.
I agree though that the KBG is an unlikely "home world" per se because it's apparently a dead world with only rocks, dust and sand as far as the eye reaches.
A part of KH's donked up cosmology is that it treats locations as worlds. I very much doubt that LoD encompasses the whole world and much like how I assume that "France" exists outside of Beast's Castle I don't think the transformation of LoD to CO literally transformed the whole "planet". Just that particular property.
Heh, I guess that's because in each world, even the original ones, we only get one or at most two distinct locations to explore per game. That may give sometimes the impression that there is nothing more in that world.
Twilight Town is also a good example as while we can explore two districts when counting Sunset Terrace, from the top of the Clock Tower you can see that the town itself is still much bigger than that what you actually can explore and while the town is apparently surrounded by forest (how that goes with the notion that Twilight Town is supposed to have a beach is beyond me) it does not show what is possibly inside or beyond the forest.