I tried a number of techniques to resolve the problem, such as accepting the change in each possible method (by clicking the bubble, from the reviewing pane, and from the reviewing toolbar), by saving the document as a new file, and by deleting the change and re-doing the change. Finally, in my desperation, I asked robg via iChat, and (after suggesting the same operations I had tried), he suggested that I copy the entire text of the document and paste it into a new file.
This worked! Not only did it remove the change, but the copy retained my document's footer (which I was worried would not be copied). I had never had this problem before, but should it happen again, I can save a lot of time.
[robg adds: This 'copy all and paste to a new doc' trick is one I've used with Word a lot in the past, and its utility extends beyond tracked changes issues. I've had other situations where I wasn't able to modify a style, or a table just wouldn't format properly. Pretty much any time I have a Word issue that seems immune to the normal solutions, I try a fresh start via copy and paste, and this nearly always seems to do the trick.]

