During ＂Operation, Maintenance, and Performance Monitoring＂ defined as the 11th Phase in EN50126-1, the completed and accepted Railway or Mass Rapid Transit Electrical and Mechanical System will be handed over to the Operation Authority. When facing equipment's / components' problems, such as costs, difficulty in procurement, unexpected reliability performance, etc., the aforesaid standard should be followed to adopt locally-manufactured and competitive parts. According to the standard, an ＂Impact Analysis＂ shall be performed prior to the planned replacement of the existing equipment / component, also called as design changes. Gap analysis and the impact analysis are to analyze the reliability and maintainability of new equipment / component, to assess whether the System Assurance requirements applied during design phase can be complied by the system / subsystem / sub-subsystem, and finally to ensure no impact will be brought to the Revenue Service during installation and tests of the new equipment / components. This paper is to describe, with flowcharts, how to introduce and to perform reliability-related analyses against new equipment / component by following EN50126-1. With a case study of the door system of the High-capacity MRT Rolling Stock, this paper is to describe the full replacement process for its components.