Automessage differentiation between free blogs and supporters

What is the best way to send different automatic messages to those who have free blogs and those who have upgraded to supporter.

When someone signs up for a free blog I want to send them automated mails encouraging them to upgrade and promises of how great it will be.

Once someone becomes a supporter then they can get more helpful automated emails and there is no longer a need to ask them to upgrade.

I cannot see how to differentiate between the two groups with automated mails. Is there some combination of method using automessage and/or mailchimp which makes this possible. I am sure there must be but can't figure it out. Any ideas anyone?