When editing the web map, you can override the scale visibility of the map service through the context menu on the layer (small arrow next to layer name). There is a scale visibility option. This will allow you to set the scale range *narrower* than that of the service. You can't set it beyond the values that the service supports.
I would think that you could just update your map service with the new scale range. If the layer in the web map doesn't update to reflect that range, go to scale visibility again and set it. It may be that your web map has overridden the default of the map service already.
Thanks,
Mike