You don't need your original disk at all. Just your version 8 serial number.
If you install Acrobat XI upgrade on a computer with v.8 already installed then v.8 should be detected by the XI installer.
If you install Acrobat XI upgrade on a computer with no Adobe software installed then the installer will ask for the serial number of an earlier qualifying version. You enter your v.8 serial number and the XI upgrade installation will continue.