I’ve upgraded one of my AWS machines from Ubuntu 18.04 LTS to Ubuntu 20.04 LTS.
The process was really straightforward, basically run:
sudo apt update sudp apt upgrade
Then Reboot in order to load the last kernel.
And ask two or three questions in different moments.
After, reboot, and that’s it.
All my Firewall rules, were kept, the services were restarted as they were available, or deferred to be executed when the service is reinstalled in case of dependencies (like for PHP, which was upgraded before Apache) and I’ve not found anything out of place, by the moment. The Kernels were special, with Amazon customization too.
I also recommend you to make sure to disable your Apache directory browsing, if you had like that, as new software install may have enabled it:
a2dismod autoindex systemctl restart apache2
I always recommend, for Production, to run the Ubuntu LTS version.