Delete the database icon from the user's workspace and re-add it. Deleting the database will cause all the Private Views to be removed. When the user re-opens the database, new Private Views (with the updated design) will be created.
You can do this via a macro that is sent to users via email:
@Command([WindowWorkspace]); @Command([FileOpenDatabase];........); @Command([FileCloseWindow]); @PostedCommand([EditClear]); @PostedCommand([FileOpenDatabase];.......)