Which ASME B31 piping code should be used for changing the rating of hydrogen piping systems?

The requirements of the code used for the original construction apply. The piping may meet the requirements of more than one code. In which case, the code used for changing the rating may be different than the original code of construction. In any case, the re-rated system should meet all of the requirements of the selected code. Note that if the original proof test of the system was not high enough meet the requirement for the new service, the piping will have to be tested at the higher pressure.