My understanding is that all the original elements used to create the Original Trilogy were saved (or at least all the ones that WERE saved) and went over to Disney with the sale. So yes, if they wanted to recomposite original effects over the top of the restored '97 negatives, they could do that.
But even if those elements didn’t exist anymore, considering the level at which VFX have progressed since 1997, they could probably just duplicate what the original plates looked like in the computer. If VFX houses can de-age Robert De Niro and resurrect Peter Cushing, they can probably figure out how to re-comp/fake-comp 1977-era VFX at 4K.
But yes: These aren’t the 2011 blu-rays. Many of the same changes applied to the blu-rays are still here, but the 4K restoration Reliance did in 2012 was done by re-scanning the 1997 negatives.
But as always, the question isn’t whether Disney CAN do something, but if they actually WANT to. And then it’s a question of whether Lucas won’t mind - because the execs at Lucasfilm and Disney do still care about whether or not they piss him off, and they don’t want to do that if they don’t have to. Hence Lucasfilm and the MPAA deferring to him in the case of that 70mm print being screened earlier this year.