See http://www.resources.autodesk.com/med/Autodesk_Softimage_Promotion

From the FAQ, here are some details:

From February 1, 2009 through January 15, 2010, Softimge XSI customers will have the option to migrate their existing products to the then current Autodesk Softimage products at 50% off SRP when purchased with Autodesk Subscription.

All migration purchases will be fulfilled with Autodesk network products. If standalone licenses are desired, a request to downgrade to standalone licenses should be noted on the order.