What are some alternatives to upgrading without using the standard upgrade system? Suppose for example that I wanted to upgrade an Ubuntu installation on a machine with a poor Internet connection. What would my options be? Could I just use a standard Ubuntu disk to upgrade this machine? If I already have a standard Ubuntu disk and want to use that, could I do a clean install without wiping data?
You can do an installation of a newer version of Ubuntu over top of an existing installation. You'll lose all of your (non-local ) system files and applications, but it will preserve everything in /home.
Select the advanced partitioning option from the menu of either the desktop CD installer or the alternate CD installer. Set the mountpoint of your existing root partition to / and make sure the format box is not checked. Repeat these steps for your home partition, if you have one.
1: Where local system directories would be /usr/src, /usr/local, and /var/local