My not simply use substitution? no need to change the workflow, and all workitems which belong to the user who left are immediately shown in the new user's business workplace. any new workitem which will be crated for the user who left will also be seen.
https://help.sap.com/saphelp_nw70ehp2/helpdata/en/8d/25f558454311d189430000e829fbbd/content.htm
You can set the substitution for other users in transaction RMPS_SET_SUBSTITUTE.