This would require a client, you couldn't do this with a web based interface directly to the server. If the main issue is to keep the content a secret from the server then all the clients can just use the same AES key to encrypt/decrypt.
The main reason of th the AES keys is only to keep the content hidden from the server. For large forums it would be safe to assume that the key would be leaked very quickly. However I image that there will be many forums, for small forums (say 10 members), it quite feasible that the ASE key will be kept secret for an extended period of time.
The latest AES key will encrypt an archive of the previous key. Having the latest key will gain one access to the entire history of post within that forum. When the key is changed, only the new posts are no longer available.
This would require a custom client and is kind of p2p in nature. So the only reason the server is there is to host the content and nothing else, this would make the server very easy to implement and the client would be more difficult.
The server's role is to check that a member posting is doing what the 'forum owner' declares a member enrolled in that group can do. For example, a member of a 'user' level group can add new content, however cannot remove other users posts. A member of the 'moderators' group can remove 'users' posts. A member of a 'admin' group can do virtualy anything to the database.
A plugin to Firefox would be the most natural way to implement this sort of forum, the client software should ask for a private pgp key, that key will be used generate all the other private keys used within the forum. The server can contain an file that contains a encrypted archive of all the private information generated by each user's client. This archive is used so that when the user clears her computer history (other than her private pgp key), she can still recover all her forum memberships.