You don't mention what avast version you are using, Home or Pro ?
Changing your email should have no impact on avast 4 Home as it is only required to initially send you your registration key, when you re-register after 14 months, then you input your new email address.
As far as the forum goes, click on the Profile button and change the email you have there for the new one.
I wil leave anything related to the Pro version unless you confirm that is what you have.