Adobe Flash Professional CS5 -- which version buy: FULL or UPGRADE?

Arie is correct. You don't need to prove that you have a qualifying older version in order to purchase the upgrade, but you do need to have one installed for the upgrade to work. With older versions of Adobe products you could insert the disc for the older version and that would do. But for several releases now you've had to have the older version actually installed and activated for the upgrade to work.