So, should you uninstall Komplete Elements before you install the full Komplete 8?

No.

It doesn’t say anything in the manual / installation instructions, but the readme.txt that you will find on the installer DVD states:

– If you install Komplete 8 on a computer with a previous Komplete Elements installation, don’t uninstall Komplete Elements.

However, I would recommend you make a backup of any projects you’ve created in any NI software before you upgrade…