Background Information
To maintain the historical record in the project, you should not reassign completed work from the original resource to a new resource. Instead, completed work should remain assigned to the resource that performed the work, and you should only assign uncompleted work to the new resource.
Resolution
The project manager of the project in question should complete the following steps:
1. Apply the Resource Usage view and expand all of the task assignments for each resource
2. Click the Filter pick list and select the Work Incomplete filter
NOTE: When you complete these first two steps, Microsoft Project displays all resources and any of their task assignments that are not complete. This includes task assignments that are either in-progress or have not yet started.
3. Locate the resource whose uncompleted assignments you wish to reassign to someone else
4. Select all of the assignment row headers for that resource (the blank gray boxes to the left of each task), then release the mouse button
5. Click and hold the selected assignment row headers for that resource, and then drag them to a position just below the name of the new resource to which they will be assigned
6. Release the mouse button
Completing the last set of steps causes Microsoft Project to reassign only the uncompleted portion of each task to the new resource.