Changing destination user

Just in case you aren't aware there are a few out of the box user interface methods that enable a task to be redirected that depending on the business requirements may meet your needs without having to resort to code.

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• In their K2 worklist (SharePoint worklist webpart or the K2 Workspace worklist) the assigned user could redirect a task or delegate (basically share a task) to another user. 
  
• if enabled a manager of a user can redirect tasks of their direct reports
  
• a workflow administrator can also redirect tasks in the K2 Workspace in the management section
  

Another possible scenario is to set an escalation on the client activity that would redirect the task to another user or group after a period of time (static or dynamic) or on a particular date.

Hopefully that might give you some other options... if those user interface options don't meet your requirements, as cyclops posted, you can use the APIs to accomplish it.

Tim