An upgrade procedure for an application system without downtime is presented. The upgrade procedure includes a mix of a rolling upgrade of application servers, shadow systems, and record and replay mechanisms that employ transformation and locking, for the upgrade of the applications on the application system. Application servers are upgraded one after another. A target version of the system is simultaneously added to the original system and a shadow, or target, system. Data changes are not only carried out in the original system, but are also carried out in the target system, so that a customer realizes no downtime for their data requests to the applications.