I was trying to update an ad group title and found that you can't do it. If you try AdCenter says you have to pause your group first then make the change. However, if you pause your ad group and then try you still get this message.
Still happening to me. I even tried it on an ad group I have had paused for a couple days just to see if for some reason I needed to let the thing dehydrate or something. I am still getting the error, "To change and ad group that has already been submitted, first please pause that ad group. Then make your changes."
So it looks like the error mesage text should be updated: "To change an ad group that has already been submitted, first please pause that Campaign. Then make your changes."