You can try the following, but there are no guarantees that it will work.
Success or failure depends on your setup and expectations.
If you have a separate home partition, just install gOS and re-use your home
partition with the same user name. Do NOT format the home partition. Most people
do not have a separate partition for home, but have it nested in root on one
partition. This is one example of why it is a good practice to have separate
home and root partitions. It makes re-installation a breeze.
Assuming that you do NOT have a separate home partition, all is not lost, but it
means extra steps. Back up your whole user folder to a usb drive/key, another
hard drive or DVD. Do this while logged on to Ubuntu as it will preven problems
with permissions. Then install gOS, completely replacing Ubuntu (there is no
8.01, BTW, just 8.04 and 8.10). Then copy all of the files from the backed up
user folder to your new home folder. When prompted overwrite any files or
folders.
Things to know: Ubuntu uses Gnome as its desktop manager and gOS uses
Enlightenment or Gnome, dpending on the version. Any look and feel settings will
not be saved, if you have the Enlightenment version. If you are short on space
you do not need to import everything since you cannot use in gOS such as the
.gnome2 folder. You can find out which desktop manager is in your version of gOS
by checking it against whatDistrowatch or Wikipedia give as specs. The method I
gave is just the quick and dirty method. You can fine tune it by backing up only
what you want or need.
You can simplify things in the future by re-partitioning so that you have a
separate home and root partition. To do this follow these steps in Ubuntu or
gOS: http://www.psychocats.net/ubuntu/separatehome