Workflow - assign task - task completed - need to re assign

Question

Hello all,

Scenario: A list item is created- workflow kick starts - creates a task in a task list - a user can assign the task to a member in SharePoint Group [acheived through task edit forms] .

Here I am able to assign the task to a user and set the assignment workflow status to complete as user has assigned the task.

Now due to some reason if the user wants to change the person to which the task is assigned, when I use the same workflow and its status is complete so it is throwing error workflow is locked and cannot be edited.

I am using a Sequential workflow built as a feature using WSP builder.

Any suggestions are welcome.

Thanks