A method and corresponding system upgrades a live computer system. The method begins with migrating a copy of data from a first system to a communicatively coupled second system at a first time. Next, at one or more subsequent times, a copy of changed data is migrated from the first system to the second system until a threshold is reached. Finally, in response to reaching the threshold, a copy of the remaining changed data is migrated from the first system to the second system. A further embodiment of the method comprises: disabling access to the first system during the migration of data at the first time, enabling and maintaining access to the first system at the one or more subsequent times during the migration of the copy of changed data, and disabling access to the first system during the migration of the copy of the remaining changed data. In this manner, the system being upgraded is still highly available throughout the upgrade process.