They literally went from 大鬼族 to 鬼人.. so from "Large Oni" to "Oni-men". Everything around this points to them becoming more human.
What brings to question with all this naming business, is that Rimuru gave Veldora a surname without collapsing. Now, either surnames are relatively worthless, or this was an oversight. (Yes, there were six ogres compared with one dragon, but surely that's hardly comparable)