Didn't work. Replacing the lock cylinder actually made the problem worse. Just before I did the replacement, I tried the ignition and it worked. (That's the problem; sometimes it works and sometimes not.) I replaced the lock cylinder and tried turning the new key. It failed. The engine did not start.
I returned the old lock cylinder to its place, and it worked.
I had some difficulty taking out the new cylinder, so I removed the plastic steering column bottom cover (I made a cut above where the key goes; see pic) to get a better look.
The lock cylinder is unfastened by depressing the pin in the bottom through a hole in the lock cylinder casing.
Anyway, although I'm so far not successful, I can now suspect the problem has something to to with the electrical connection to the lock cylinder.
I hope I can return the new lock cylinder. It's from the local O'Reilly and they may be fussy about accepting returns of electrical parts.