Mr. Prashant Ingawale has very well laid out the procedure that is required to be followed by an employer regarding the notice of change. It will definitely help you better understand the criteria and develop your answer. In addition to this, I also want to add that as far as convincing employees about the proposed increase in the appraisal period is concerned, they can be assured monthly or bi-monthly meetings with their immediate superior (boss, supervisor, or manager) so that they keep getting informal and periodic reviews of their performance and thus make improvements accordingly. These informal meetings can also serve as a platform for employees to provide their feedback about company operations and resources that are needed to perform their respective jobs better.
Often, companies conduct performance reviews annually to save time and money on the entire procedure, but such short-periodic meetings not only make the employees feel involved, but also help enhance subordinate-superior relationships.
Hope this helps. Also, when confronted with such questions, you can visualize yourself as an employee and try to figure out what would satisfy you if you were one of those affected employees...
Regards,
Purva