1. A method comprising: initiating a zero downtime upgrade procedure upgrading a source system to a target system that prepares software for the target system in parallel to the operation of the source system; renaming source tables of the source system; cloning the renamed source tables for the target system; equipping the renamed source tables with database triggers to transfer data from the renamed source tables to the cloned tables used by the source system according to a source database schema; generating the target tables comprising updated software content for later use by the target system while the source tables are used via a bridge database schema; and sharing tables between the source database schema accessed via the bridge database schema and the target system.