I noticed that people often come to the practice forum to test their avatars, but it isn't necessary to actually post a message (though if that's the way you prefer to do it then please continue - it causes no problem). If you already have avatar display enabled, then updates to your avatar image take effect instantly everywhere. Simply view one of your own messages (hit refresh if the message was already in your browser window) and you'll see the new avatar. Or you can reply to any message and then hit the Preview button, which will display your (null) post, but will also include your avatar. Then, instead of posting the null message, simply hit the back button on your browser.
Updates to whether your avatar should be displayed or not do not necessarily take effect instantly. If you turn it off, that takes effect instantly everywhere. If you turn it on, it will only appear on new message pages as they are built, ie, as people post new messages.
--Percy