On: Ubuntu 16.04 (Xenial Xerus)

[Rezo]

That is correct, upgrades are not enabled between LTS releases at this time, also for servers AFAIK.

Either way, personally I would never upgrade a server in place these days. Treat your servers like cattle not pets: Rebuild from new base image, validate, put into LB/proxy, terminate old stack.

[poooogles → Rezo]

This. Our upgrade path was changing a variable in our packer config.

Literally the easiest upgrade ever.