Parallel running is one of the ways to change from an existing system to a new one.
During changeover, a new system and an existing system run side by side. To input the same data and perform the same process

Process (computing)

In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...

es, compare their output

Output

Output is the term denoting either an exit or changes which exit a system and which activate/modify a process. It is an abstract concept, used in the modeling, system design and system exploitation.-In control theory:...

 and prove the reliability

Reliability engineering

Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...

 of the new system. If the new system is accepted, the existing system will stop running and will be replaced by the new one.

The practical example of parallel running in human resource management

Human resource management

Human Resource Management is the management of an organization's employees. While human resource management is sometimes referred to as a "soft" management skill, effective practice within an organization requires a strategic focus to ensure that people resources can facilitate the achievement of...

 is job placement. A new staff and an old staff work for the same job. If the new staff's performance is ok, the existing staff may not be needed any more, and will be replaced. This is different from direct changeover and phased implementation because parallel running requires two systems working at the same time.

Other than this, there is direct changeover

Direct changeover

Direct changeover is one of the methods to change from an existing system to a new one. This is the simplest methods of changeover. Imagine a new checkout system for a supermarket...

 and phased implementation

Phased implementation

Phased implementation is a method of changing from an existing system to a new one. Phased implementation is a changeover process that takes place in stages. Think of a supermarket for example. Imagine that only the vegetable section is changed over to the new system while the rest carries on with...

.