My understanding is it’s just the blu-ray/4k. In the comparisons I saw it just looks to me that they just zoomed out a little and changed the lighting. Didn’t look much better to me.
I saw it in theaters and just rewatched it on Hulu, before those comments about changing it came out. I don't remember anything that different, so I couldn't say for sure if anything was changed in the streaming version. But I'm also one of the people that didn't have problems with the FX of Rook in the first place. I just think of it as anything that doesn't look "real" about him being due to him being massively damaged. And I think about the original Alien, where there's an obvious film cut between setting down the prosthetic head of Rook and the shot with Holm sticking his head through a table. Even with the limitations of the FX tech of the time, they could've shot it differently than they did so that the cut wasn't so obvious. It may affect your 'suspension of disbelief,' which is required for any viewing of fiction, but sometimes you just have to accept the unbelievable and not fixate on it. Ultimately, the FX of Rook in 'Romulus' didn't affect my enjoyment of the film, and I actually liked the way they tied the two movies together.