To upgrade from 6.x to 7.x just follow these steps:

* Back up your database.

* Update your 6.x site to the very latest versions of all modules.

* Back up your database.

* Update to the suggested modules of 7.x, please bear in mind that
  og module has to be >= 2.x

* Run update.php

* If everything works, you may delete og_mailinglist table from 
  database since it is no longer needed, but is not automatically removed.
