Sometimes a resource that had been working on a task needs to be pulled off that task. This could be because the resource is required on another task or project, is sick, or has left the company. Project provides a way to replace a resource on an assignment even if it's already done some actual work. The Replace button on the Assign Resources dialog takes care of this functionality. The great part about it is that you can still keep track of the fact that the old resource did some of the work, whereas the new resource did the rest.
Replace a Resource
Select the task to which the resource that needs to be replaced is assigned.
Click Tools, Assign Resources.
Select that resource in the Assign Resources dialog.
Click the Replace button.
In the Replace Resource dialog that appears, select the resource that should replace the old resource.
Click OK.
Notice that Brian is still assigned to the task, but only for the days where he already had Actual Work.
Steve now has the Remaining Work for the assignment.
Select the task to which the resource that needs to be replaced is assigned. 
Click Tools, Assign Resources. 
Select that resource in the Assign Resources dialog. 
Click the Replace button. 
In the Replace Resource dialog that appears, select the resource that should replace the old resource. 
Click OK. 
Notice that Brian is still assigned to the task, but only for the days where he already had Actual Work. 
Steve now has the Remaining Work for the assignment. 
