So far, I can’t get it working either. This works on a live site, I haven’t been able to get it working in my development environment. The SSO params that are sent to Discourse now have a filter applied to them, so you don’t need to override the plugin’s code.
Adding something like this to your functions.php file should work.
add_filter( 'wpdc_sso_params', 'my_namespace_force_avatar_update' );
function my_namespace_force_avatar_update( $sso_params ) {
$sso_params['avatar_force_update'] = 'true';
return $sso_params;
}