Hi Jothi,
Okay, your original posting "How to change the assignment to task"
create the confusion. With no reason, my self and Jack are exchanging
the postings

Now answer to your updated question
It depends on how to proceed with your situation
Example:
Task A -> Resource A
New Resource Name: Resource B
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Scenario 1:
If Resource A already worked on Task A and actual work associated with
this task and you want have visibility of Resource A's actual work on
project plan.
Solution: Create new task and assigned the Resource B; assign work;
Also make remaining work is zero for Resource A.
Alternatively, you can use copy assignments method in Resource Usage
view. This method copy the actual work and as well as work.
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Scenario 2: You have Task A and you didn't have any actual work from
Resource A
Solution: On Gantt chart view, just pick the Resource B from dropdown
against Task A
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Thanks
Chak
http://www.epmcentral.com