One of the challenges facing the designer of an upgrade for a hydroelectric power plant is finding accurate information to base the upgrade solution to be implemented. In some cases, the data available from the unit is vague or incomplete. The lack of data could cause wrong decisions in the design that could affect the performance of the new equipment or even compromise the plant's safety. Before the design stage, the pre-shutdown testing of the units is a powerful tool that allows the engineer to obtain relevant data from the unit. Also, these tests could help discover hidden problems in the equipment to be upgraded and avoid risk for unit safety, reduced expected performance, and unexpected costs during the retrofit project. This paper describes real examples from different projects pre-shutdown testing. During these tests and the data processing after it, designers found facts regarding the units to be upgraded that help produces safety designs and insure the expected performance for the equipment after the rehab. These real-life project findings are described here to prove the importance of pre-shutdown testing before units overhaul.