I'm not sure why it's so hard to get this information from Symantec, but here you go.
Based on a response from CSR Rep Linda, you can use a CD, Title page from a manual, Email Confirmation from a Download purchase, a printout of your Help/About Screen, a screen shot of your previous version (not sure what that one is) or your receipt from the older purchase.
I've included her message verbatim (bolding by me) for reference. Hope this helps you all and ends the need for anyone to contact CSRs for clarification on a poorly worded rebate form (someone really needs to fix that).
Because to qualify for an upgrade offer you had to previously own a stand-alone or pre-loaded version of Norton or Symantec software or another company?s antivirus, firewall, antispam, antispyware, utility, or back-up software product, the Proof of Previous Ownership requirements can be any of the following:
1. A copy of the CD or diskette 1 from the previous software 2. The title page from the previous software's manual
3. A confirmation e-mail from a download purchase of the previous software
4. Print out of Help/About screen
5. Print out of screen shots of previous versions
If you do not have the manual or a CD from your previous version, any of the other options will be accepted. A purchase Receipt/Invoice from the purchase of the previous version will also be accepted.