I reinstalled from the latest ISO two of my 4 machines. Overall it was incredibly smooth!
I use multiple vpn clients, so I backed up their settings (in /etc/ppp and /etc/vpnc.)
I have separate root home and var partitions on all my machines. I selected to use existing partitions and only format root. I only had one user account on each machine. I created a user account with the same name, and used the same passwords for that user account and root account as before.
Post installations, all my setting were preserved! LibreOffice Thunderbird and Firefox worked without a hitch (even updated their plugins correctly.) I had to install some other applications and utilities that are not part of the default install (e.g. openconnect.)
I usually am not very good in giving compliments... this upgrade process is an amazing feat!