Step-by-step guide
You may be tempted to try it like this:
% grib_set -s packingType=grid_simple second_order.grib simple.grib
But this will not work and issue errors re codedValues and bitsPerValue! You need to use the repacking option as shown, adding the "-r" option to repack the message:
% grib_set -r -s packingType=grid_simple second_order.grib simple.grib
Also see the code example: grib_precision