I followed this wonderful guide: https://www.cyberciti.biz/faq/aws-lightsail-upgrade-ubuntu-16-04-lts-to-18-04-lts/
I rebooted without taking note of an sshd_config problem. Of course I was no longer able to ssh into the instance.
I reverted to an earlier snapshot
Started the process over again.
After the update finished and asked me to reboot I refused and then fixed my sshd_config file:
$ sudo sed -i ‘s/^Ciphers .*/Ciphers +aes256-cbc,aes192-cbc,aes128-cbc/’ /etc/ssh/sshd_config
Now I get:
Welcome to Ubuntu 18.04.2 LTS (GNU/Linux 4.15.0-1035-aws x86_64)
when I use ssh.
Which is great! I upgraded to Ubuntu 18.04 LTS and was able to head off a breaking change to SSH that happens with the upgrade.