I have attached the script I modified to automate this process via cron.
It is essentially a stripped down version of the batchupgrade.php script.
Since it runs the exact same process as if you had clicked the "Upgrade All Assignments" button through the web interface I am hoping there won't be any issues.
If you decide to use this script remember, it is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.