I have seen this option in 18.04 somewhere. Perhaps up to 18.04.2. Where, if you have an 18.04 already in the machine, the installer will detect it and give an option to replace the existing 18.04 with a fresh 18.04.
But recently, when I tried with 18.04.3 live USB, the option was limited to erase the whole disk and then install Ubuntu. The machine already had a working 18.04 which had no mention in the installer.
I am hoping to see that we can somehow reset 20.04 through Live USB or at least have the old Erase 20.04 and install ( 20.04 ) other than only options for full disk erase.
You seem to be asking for options already there, ie. “Something Else” (or Manual Partitioning using calamares installer used by Lubuntu)
If you select that option, use your existing partitions & do not format you partition(s), you’ll have the installer (assuming desktop)
your added packages will be noted
your system directories erased
new system is installed
your added packages are installed (if available in your new release
no changes are made to your user data
This has existed for years, and does require you to not-format. Won’t this do what you already want?
It’s a great way to re-install your system if you mucked something up (in system directories; if it was a user-config issue, those issues will remain as user directories are not touched). It’s also a great means to switch from one flavor to another; I’ve been using this since at least 11.04, probably existed well before that too.
Notes: If it’s a user config issue; creating a new account on your existing system will not have that issue; thus it’s detectable before you re-install. Also if you manually installed or re-installed packages (eg. sudo apt install --reinstall..) they’ll be noted as manually installed by package tools rather than being related to the installed desktop (not a problem IF you’re re-installing the same desktop, but can cause clutter if using this method to switch from one desktop to another)
What I am trying to say is, there was a GUI based, one-click option to reinstall existing 18.04 without erasing the whole disk. It was there for 18.04 and I was hoping that it will come back for 20.04.
Once you get into Something else, you can mess up a lot of things. Considering that a GUI based option alongside Something else was a great help for a lot of users.