We do something similar here except we change the Status to ASSIGNED. We have a workflow rule that fires a field update that calls an incident template that sets the status to ASSIGNED. Here is the rule we created:
(BMCServiceDesk__Status_ID__c = "OPENED" && ISCHANGED( BMCServiceDesk__FKOpenBy__c)&& NOT( ISBLANK(BMCServiceDesk__FKOpenBy__c)))
Hope that helps.