Do I have to download the LiveCD/DVD every time to upgrade?
No, Sabayon is a rolling release. However in some cases upgrading via live DVD may prove more time efficent. Entropy: The equo command makes upgrading easy – see Entropy. Portage: Portage is recommended for advanced users that prefer a higher degree of customization. That being said, Gentoo documentation is quite extensive and helpful for those wanting to dive into it. Always backup your files before doing anything. My personal opinion: backup, fresh install, restore backup.