I got a message telling me that the database schema was several versions out of date, and instructing me to run either mythtv-setup or mythbackend in order to update the database. I tried both (from a console) and both failed (with mythtv-setup trying to start mythbackend). Eventually, I discovered the first (of multiple) problems.
Problem 1: Time zone data
Somewhere along the line I got a message that MySQL, the database program used by MythTV, was lacking time zone data. I followed some instructions I found, and fixed it by running the command
mysql_tzinfo_to_sql /usr/share/zoneinfo | mysql -uroot -p mysql
supplying a password when prompted. After that, I was able to run mythtv-setup and mythfilldatabase. Apparently mythtv-setup automatically updated/fixed the database.
Problem 2: Security PIN
In mythtv-setup (general settings) I discovered that the security PIN was blank. According to the prompt, that will prevent any front end from connecting to the back end. Per the prompt, I changed it to 0000, allowing all front ends to connect. (I only have one, on the same machine as the back end, so that's fine.)
The IP address for the back end was correct, but since I have no remote front ends, I changed both the local and master addresses to 127.0.0.1. That was probably unnecessary, but I mention it just in case it had any unintended effect. [Update: In hindsight, I realized that might interfere with my using MythWeb to program recording schedules from other devices on my home LAN, so I switched both the local and master addresses back to the machine's static unrouteable IP on the local LAN, with no harm done.]
Problem 3: No Back End Service
Even after successfully running the setup program, the front end still could not connect to the back end .. because the back end was not running ... because it failed silently every time it was started. The good news was that I could start the back end manually (running mythbackend in a console), after which the front end could connect. It came as quite a relief to see that recordings, channel information and recording schedules had all survived the upgrade. Still, you can't do scheduled recordings very effectively is someone has to log in and start the back end manually each time.
Fortunately, I was able to get help from Thomas Mashos in the Mythbuntu Community on Google+. I could start the back end manually because my personal MythTV configuration file (~/.mythtv/config.xml) survived the upgrade intact. The system configuration file (/etc/mythtv/config.xml, also symlinked to /home/mythtv/.mythtv/config.xml) must have been replaced by what amounted to a placeholder. The host, user name and database name fields were empty; the password field was filled in incorrectly (or maybe that was the correct password for a null user access a null database on a null system?). I edited those four entries to match my config file and (woo-hoo!) the back end resumed playing nicely.
Problem 4: No Sound
Once the back end was running, I played back part of a saved recording to verify that things were working okay. The good news was that the video played fine. The bad news was that there was no audio. This turned out not to be limited to MythTV: I couldn't get the system to make sounds, period. (In contrast, the system had no trouble getting me to make sounds. Very, very aggravated sounds.)
I searched hither and yon on Google, and discovered that a nontrivial number of users have (or had) sound problems, many of them as a result of the upgrade to Mythbuntu 14.04. I've lost count of the number of options I tried, including purging the alsa-base, linux-sound-base, pulseaudio and pulseaudio-module-X11 packages (all at the same time), reinstalling them (I even deleted the archived copies and forced the package manager to download them again), and of course rebooting.
Eventually I stumbled on a fix (woo-hoo!). I had previously poked around in the ALSA mixer (by running alsamixer in a terminal) and unmuted the obvious suspects. I gave it one more try, disabling Auto-Mute Mode (because the last thing I want right now is something muting the sound again) and playing with settings I didn't understand. It turns out that unmuting both "S/PDIF" and "S/PDIF Default PCM" restored the sound. I have no idea what they stand for, let alone why they were muted. From aplay -L, I can see that they're associated with my NVidia graphics card (which also handles sound). So are eight bazillion other things, and I'm not sure why this one is special.
Problem 4.5: No Sound After Reboot
One might hope that unmuting the two settings above would be a permanent fix. Not quite. After a reboot, "S/PDIF Default PCM" remains unmuted but <sigh>"S/PDIF" finds itself muted again</sigh>.
In alsamixer (in a terminal), I unmuted both, then ran sudo alsactl store to store the correct settings and added alsactl restore to /etc/rc.local, which should hypothetically be run each time the system boots (but not, I believe, during reboots). The command works fine from a terminal, but after a full shutdown/start I find S/PDIF once again clobbered; so either /etc/rc.local is not running or, more likely, it restores the correct sound settings before the evil program that's muting S/PDIF gets executed.
Just for grins, after a reboot (and without restoring the ALSA settings, i.e., with a silent PC), I went into MythTV's audio setup menu. It showed a sizable number of options that all mentioned the correct NVidia card and matched what I saw when I ran aplay -L. The default choice produced no sound. I switched to a different setting (from "dmix" to "front", as I recall), ran the sound test and it worked. Unfortunately, while the setting survived a reboot, sound did not. So I tried again, switching to "ALSA:iec958:CARD=NVidia,DEV=0", and that setting works. It has survived three restarts, so I'm a cautiously optimistic that my long ordeal (most of two days chewed up fixing this) is finally over -- pending the next upgrade, of course.
Problem 5: [update] no MythWeb
After posting this, I discovered that MythWeb was not running. This turns out to be a known bug, due to a difference in the Apache versions used by Mythbuntu 14.04 and its predecessor. The fix specified in the bug report, which I repeat here, worked and MythWeb now runs as expected.
$ sudo rm /etc/apache2/ $ sudo dpkg-reconfigure mythweb $ sudo /etc/init.d/apache2 start