I've spent over a month on this problem so now it's time to ask for help.
I've stayed with the LTS releases since I installed Ubuntu about 10-15 years or so ago. I use POP mail in Thunderbird. I use a Crucial SSD for the root drive and 4 more for cloned backups. Crucial recommends 4 years use according to a tech I talked to early in this adventure. My purchase receipt says I bought them 5 years ago. My bad for not paying attention to the elapsed time. A drive started returning bad data. By the time it had gotten bad enough to prevent the machine from booting, all 5 of my drives were contaminated. Thus no opportunity to export my 16.04 TB profile.
My first thought was to re-install 16.04 long enough to export my profile. Search as we may, we could not find the install disk (I install from a thumb drive but backup to a DVD also). All critical data goes into a fireproof safe but for some reason, this DVD didn't.
So I searched around and found a forum (not this one) comment that 18.04 should be able to import. I installed it. No, it can't import. I found a vague comment that the next minor release would have that capability. It did not. Someone has written a conversion utility independent of TB. It gets about half way through my profile and pukes.
Soooo, I dropped off the LTS train and installed version 19. Still no import ability but I found an import-export plugin and a fork of same. Both reported success but TB can't see the resultant profile.
I'm at my wit's end. I have 10-15 years' worth of business correspondence in there (all divided into annual folders) so I really need to recover this profile.