I believe that the "add a block" block gets added to the "default region". If you look at the config.php file in the root directory of your folder you should see an array called $theme->layouts.
On the pages that have two (or more) block regions
regions' => array('side-pre', 'side-post'),
You can change the default region to the opposite one:
'defaultregion' => 'side-post',
'defaultregion' => 'side-pre',
At least that's the general principal.