I, too, have a server running 16.04 that I'm afraid to update. It currently has an uptime of 1281 days... at this point I'd feel bad rebooting it
What is there to be afraid of? Don't you have backups?
Also, debian/ubuntu systems can easily be setup to auto update and reboot on a regular basis, leaving you manual maintenance only for the larger version upgrades.
My oldest server is on 8.04.
dd filesystem to another machine then boot it up with an emulator like qemu and do a trial run
Be careful if you have anything that autostarts that reaches out