Ah ha! Thanks for the tip about el.db being the old one, and el-v1.db being the new one

I took a backup of ~/.rtcomm-eventlogger/, and then deleted the el-v1.db + journal files. I then rebooted. After the reboot, the conversations app took a long time to start. However, when it had come up, all my old SMSs were there!

I've sent a new test sms, and that's come in too

Seems I've lost a couple of messages that came in post-upgrade, pre el-v1.db delete, but at least the rest is back. Also, the el-v1.db file is now a different size, which seems to indicate that the upgrade has worked this time