Changing a Due Date once the process has begun

Summary

Are we able to change a due date company wide once the process has begun?

Content

This is our first year doing this. We initially listed a due date for Self Assessments and all other steps in the process. Since it is new to all employees, we want to extend the time for the first two steps - Employee Self Assessment and Supervisor Reviews. If we do the change in the Plan Document, will that filter down to everyone that hasn't yet submitted their review? Or is this change only allowed on a person by person basis?