When a new Ubuntu version is released you have only three options.

1. Upgrade from inside the system which keeps data.
2. Download .iso and install the new one.
3. Download .iso and replace a previous installation manually to keep data. [This one is differcult to do.]

...but there is no way to run a 'clean upgrade' which upgrades the system like in 1. but would also erase all your data if you want a fresh installation. There is also no way to only save selected data...what if I only want the PPAs?

And inside the .iso there is no easy to see option to replace only the exist Ubuntu installation and import selected data.

Edit:
The installer checks for other Ubuntu installs and asks if you want their /home...their could be a drop down labelled advanced which asks if you want the sources.list and /etc and folders such as that.

The page can be skipped if the user doesn't want to copy anything. It would work similar to how Ubuntu finds pictures and videos from windows.

I personally only want to keep the software sources list and package lists. Selected data would be any user data from the old one.

The natty installer would check for the words 'maverick' in copied PPAs and replace any 'maverick' it finds with 'natty'. Similar thing for newer ubuntu versions.

In the update centre instead of 1 button asking for an upgrade you would get a dropdown menu eg:
New Ubuntu version available-Upgrade >


[....]

I looked at sources.list and I know how many repositories I usually add.

The list gets long and cluttered.

There are also some ppas that are duplicates or rely on others etc.