2011/1/19 Earnie Boyd <earnie@users.sourceforge.net>
You'll need to focus your response on the benefits of security and progressive development. Version 5 is no longer supported and Version 6 will only be minimally supported. So moving from an older version has the benefit of being able to keep up with security releases and newer technologies that were not able to be foreseen in the older versions. Also point out the improvements to the user experience which can be a savings in time to use the site which is a benefit to your client's clients.
D6 will be maintained for 1-2 years (as a usual dev cycle), but as a developer it requires more energy to keep in mind the both release branch. If i were you, I simply tell the client the support for the old branch is restricted to the security issues. For further development, his site has to be upgraded to the latest stable branch or he has to pay extra percentage for the out-of-date development. -- Aries