Please indicate some historical basis for moving an installed base of users on this kind of scale and for this kind of reason.
History is replete with examples. From the Internet Worm to Code Red, consumers do install software when they perceive either a threat or a benefit. Getting rid of spam is a HUGE benefit. Heck. What I've found so amusing is that people seem to upgrade their Microsoft systems just 'cause, with no perceived benefit, but merely protecting from Bit Rot.
Eliot