While they look alike, HomebrewersAssociation.org and the AHA Forum are actually two different sites.
When you initially register on the AHA Forum, it will send you to HomebrewersAssociation.org to register. Your user name and password gets pushed to the AHA Forum, but it only works one way. If you change your user name and/or password on the Forum, it will not update your username and/or password on HomebrewersAssociation.org. If you change things on HomebrewersAssociation.org it should carry over to the AHA Forum.
We'll be switching to a new association database in early 2011. We're looking into adding a single sign-on for all of the AHA web properties including the Homebrewopedia and Club Resources wikis at some point after the new database is in place. Hopefully sooner than later!