I think this might be a case of be careful what you wish for! As a space cowboy once said "I've got a bad feeling about this".
I'm not moving my army back to squares. I'll sell it all before i do that. Noooo freaking way. gaaaaah.
Wait, IS there a Meme god? How would one go about ... wait, obviously spreading Memes is paying tribute to the Meme god...
With the current revival of the Horus Heresy, i only wait for someone shouting "Death to the false Meme God!"
Other than including square bases in boxes of old Fantasy models repurposed for AoS, you couldn't allow any other AoS models to be included in Fantasy, as that would just ruin the whole point of trying to bring Fantasy back as a prequel to AoS. Most of the AoS races only appeared after the End Times, so to introduce them into the Fantasy world would ruin all the lore GW built up over 8th Editions.
I don't recall how much is enforced the "only round bases in AoS", but with the Old World with square bases, I'm confident that round bases will be the only ones playable in AoS. Kinda like WYSIWYG: pick the base / equipment you want, and stick to it.
Currently GW say using square bases on minis is fine, but measure to and from minis as if they were on the round bases, but of course Fantasy is still nonexistent at the moment. I wouldn't be surprised if GW become more strict on the basing rules in the future when the new Fantasy edition comes round, but you never know.
I really hope you are correct. GW "double selling" models into two games is one of my biggest fears with the WFB revival. I want WFB to maintain its previous style and aesthetic. Certain things like the new Trolls for example would be a welcome addition, but other things like SCE (which I hate) and KO (which I like) are better suited for AoS.