Did somebody say my name?
It's not so much the ethics per se (it is your own intellectual property after all), rather that once people read the update they would not get any of the changes as usually they do not go back and re-read it. Many of us go back and tweak chapters for typos, grammar etc, but usually nothing substantial or plot-changing.
You can of course edit previous updates and post something to let people know, but there is no guarantee how they would react. Also, it is different when you update the latest chapter or a chapter before the latest one.