Have you ever heard of multibooting many distros from the same partition (eg, MultiBoot USB with Grub2
Do you need to find a way to install the new Ubuntu 10.04x LTS into an existing partition as another option to multiboot?
Simple, you don't need to download or install any scripts or executables. But you should be familiar with command line.
# Prepare source
sudo mount -o loop iso/ubuntu-10.04.1-desktop-amd64.iso