When we first acquire a new system, or replace a system, we know what the operating parameters as well as what the systems purpose is. However as time progresses, what will happen is that human interaction and needs will require for the alteration or modification of the system to fit the current mission or demand. By changing operating parameters or modifying the system as a whole or one of the subsystems we can expose new risks due to the non-standard use of the system. Without a proper risk assessment, one can be put into an uneasy situation.
For example here at the flight training department we use the Piper Arrow, as our single engine complex trainer. The term Complex means an aircraft that has retractable landing gear, wing flaps, and a constant speed propeller. However the only portion that we want to focus on right now is the landing gear, since the gear retracts and stows into the fuselage during flight to improve performance it must also come down at the end of the flight to allow the airplane to land. However because of human interaction, there is the risk of the aircraft landing gear up if the pilot were to not remember to extend the gear on approach to landing. To help mitigate this risk from becoming a mishap due to our large operation, we installed radio altimeters and an aural (speech) alerting system that would alert us of the gear not being down and locked. However since this was a modification to the current gear alerting system, we had to go through multiple modifications to the aural alert system.
Thus in conclusion, the systems in use are usually either evolving as our needs for the system change, or we replace the old system with a new one. However the replacement of a system can be costly and thus modifications or additions are more attractive means of expanding the life cycle of a system. We however want to take note that we want to try and keep tabs on new risks and hazards arising from these additional subsystems.