Absolutely nobody (sane) will care if your minis are on rounds in a movement tray of the right dimensions. The only tangible reason some people are talking about rebasing is where the rounds they have are oversize for the square equivalents given how they want to proxy (say, a really big based AOS character they want to make into a unit champ) - other people talking about it just fancy having another go at their bases.
Way back we polled folks about their basing intentions, and most people said even back then that they’d be using converters and that was before we got much meaningful about the game.
It wasn’t long before we got @Pyroven 's superlative, encyclopedic thread on where to get said spacers/movement trays.
As he and GW suggest, I’ve got whole armies of hundreds of minis on 20mm squares, and I cba to change them. If your guys are on circles, it makes no difference to anyone sane.
Lastly, I’m not sure what your local gaming environment is - but we haven’t seen anyone banning the legacy PDFs, which makes sense, as they’re clearly competitive at this early stage. They were written well to be contender lists. Over time they might fall behind with campaign book units, but for core original grand army play, I don’t think you should worry about the AOS experience too much. TOW is intended to have stable, rarely-updated rules.