Yes, RW_Tools can do it and I frequently use this function to substitute one of Dick Cowan's switchers for the player locomotive in a scenario that is mainly shunting.
On the edit scenario page, the current driven consist is shown (in red). Go to the list of consists, select the consist with the same name as the current driven consist. Highlight the lead engine in the consist. Populate the Available Stock window with the engine you want to drive. Highlight that engine. Use button Swap Selected Items. Use button Save Changes to Scenario.