1. A computer system including instructions stored on a non-transitory computer-readable storage medium, the computer system comprising: a program database module configured to define, at a shadow system, a shadow program database having a structure defining an upgrade to a structure of an original program database on an original system, and having data defining an upgrade to data of the original program database on the original system; a customization upgrade module configured to select data from the original program database as the data of the shadow program database, configured to modify the selected data based on a data customization implemented in the original program database of the original system, and configured to define an upgraded program state of the shadow program database based on a combination of the structure of the shadow program database and the data of the shadow program database; a database comparator configured to identify a first portion of an original user database of the original system for upgrade, and configured to identify a second portion of the original user database of the original system for aliasing in the shadow system; a user database module configured to define, at the shadow system, a shadow user database having a structure defining an upgrade to a structure of the first portion of the original user database; a database replicator configured to perform at least one first transfer of data from the first portion of the original user database into the structure of the shadow user database during an uptime of the original system and configured to perform a final transfer of data from the first portion of the original user database into the structure of the shadow user database during downtime of the original system; and an upgrade transfer module configured to replace the first portion of the original user database with the shadow user database after the final transfer of data has been performed, and configured to replace the original program database with the shadow program database during the downtime of the original system.