Mathias, what is actually your problem? Of course forwarding a work item will have no effect for the application itself (leave request in this case). Why do you need to change the agent in PTARQ? Also other users than the one in PTARQ (PTREQ* tables) should be able to see the item to be approved in approval application (e.g. substitutes and users who the work item was forwarded).
Kind regards,
Karri