We normally use an alpha or bravo c/s designator on the reroute, so the crews will know straight away that something has changed. (unless it's just a simple level drop etc)
We also ensure the handling agents have received the new rerouted plogs, and make every effort to contact the crews direct via their company mobile phones (when they switch the things on ! :-( )or via Selcall.
This is the best way we can think of ensuring that crews don't get confused on calling for clearance !