As of February 1, 2015, Autodesk will no longer offer the
option to purchase upgrades for all noncurrent product versions. Maintenance Subscription offers the most cost-effective way to stay current. For a limited time, you can get one year at no charge.
Free Maintenance Subscription for a year on select upgrades until April 25, 2014.
As of February 1, 2015, upgrades from previous releases will only be available with Maintenance Subscription.
For more information see this link: