Hi
I know this topic is very old, but I had the same problem. I upgraded to Avast Pro for one year, and then when my year subscription expired (yesterday), I was left only with the options to repurchase a Pro license. There was no option to downgrade to Avast Free.
After about twenty minutes of searching in vain for such a button, I was ready to uninstall Avast Pro and get another Free antivirus. But when I selected the "uninstall" option from Windows 7's control panel option of "Add/Remove Programs", a message from Avast popped up asking me if I really want to uninstall Avast, and by answering "Yes" it then asked me if I want to revert back to Avast Free....
So it would seem that a lack of downgrading capabilities is a common complaint that Avast chose to solve by that cumbersome route....
I hope this helps at least someone.