With its latest version of AX, MS has given Source to Target model for upgrade. Using this model will help reducing the downtime for the actual production environment (single user mode) while upgrading from AX2009 or AX4.0. One will be able to execute the upgrade tasks on Source system (AX2009/AX4.0) without interrupting the user using the live system, and in parallel prepare the Target (AX2012) system for upgrade.