What really bothered me was that my choices were to upgrade or learn more. I didn’t want to upgrade and I wasn’t going to learn more by clicking that button. I was relieved to find that I could open the Notification Center and close the popup from there. I just launched Word (which I very rarely do) and no popup appeared. I hope I haven’t jinxed myself.
Well, I did. Following a restart, the badge is on the icon in the dock and the icon in System Preferences. (And you can tell by the start of this post that I haven’t learned to keep my big keyboard shut.)