Change evaluation

Answer:
Not all changes require evaluation. So, you need to decide which ones you'll evaluate. These are usually changes you consider important (because of their impact on the company, risks they have, costs...etc.) Therefore, once you create Change Policy (good place to define what and how you'll evaluate) - it will be clear which changes need to be evaluated.
Evaluation is made, usually, prior to the implementation as well as afterwards.
This articles can help you further:
"ITIL – Change Evaluation Process" https://advisera.com/20000academy/blog/2015/02/24/itil-change-evaluation-process/
"Three key elements of assessment and evaluation of changes according to ITIL" https://advisera.com/20000academy/blog/2015/06/30/three-key-elements-of-assessment-and-evaluation-of-changes-according-to-itil/