Changing Group Numbers is a Supplimentary Essential Variable for SMAW, SAW, GMAW/FCAW and GTAW; see variable QW-403.5. As a Supplimentary Essential Variable, this only kicks in if there are notch toughness requirements, which depends on several factors, material type, thickness, design temperature, etc. I do not have access to a B31.3 but I'm sure others do.
What thickness of 516, Gr. 70, what service and what design temperature?
PT, what jon is referring to is this: If the construction code requires notch toughness testing, than you may not use the WPS for P1 Gr 1 to weld P1 Gr 2 material. Otherwise, you may use it.
You will have to chase this down in B31.3 for your particular application.
My experience has been that when notch toughness is an issue for piping systems, it is very visible in the job specification.
Charles