I just accepted an answer to a question, but I couldn't tell that the check mark had changed.
Looking at them side-by-side, it is easier to tell the difference, e.g.:
But I could not tell that it was changing when I was toggling it. I thought that I might be imagining things until I saw that I had gained +2 for selecting an answer.
I have never had such doubt when using other SE sites. It might be helpful to other new users so I think that it is worth considering this change.