DNG users need to take on different, mutually exclusive roles throughout the lifecycle of the project

For example, here is the configuration at the beginning of the project:

• John Doe is a Preparer and NOT a Reviewer
• Sarah Smith is a Reviewer and NOT a Preparer

Here is the configuration mid-project:

• John Doe is a Reviewer and NOT a Preparer
• Sarah Smith is a Preparer and NOT a Reviewer

The need for this is driven by small teams and regulations prohibiting the same person being both a reviewer and preparer. The role changes are predictable and can be planned. It will follow a workflow where issues are identified and resolved and the identifier cannot be a resolver and vice versa, depending on how the tasks are assigned to the personnel. So for a time period, the users will have specific roles. Later on in the project, at a defined time, the users will take on different, mutually exclusive roles. Maybe this functionality could be tied to a timeline of some kind. 

Do you have any ideas for implementing this short of manually changing the user roles in the project area frequently throughout the project lifecycle?