I’ve been thinking about the Ubuntu 14 LTS to 16 LTS upgrade for awhile. It seemed like it was do-able, though comments on some web sites indicated lots of problems including bricked systems. I made sure I had a snapshot before I gave it a try.
sudo apt-get install update-manager-core
sudo do-release-upgrade
The trickiest part was resolving configuration file updates. But you can preview the differences and they were mostly changes I had made specifically so I kept the old config file.
Naturally, the upgrade broke things. Fortunately, not too badly.
PHP was the main problem. The upgrade un-installed PHP 5 without a replacement. Fortunately PHP 7 installed ok, though I had to re-visit my notes about what else to install, like php7.0-mysql, so WordPress would be happy. That took a couple of re-tries to find all the missing modules such as php7.0-mbstring, php7.0-mcrypt, php7.0-xml, php7.0-xmlrpc which weren’t a problem until BackWPup tried to run.
Looks ok now.