Do you have a WPS that was Qualified or Prequalified?
I believe that you need to look on your mill test reports for the ASTM Specification for the material you purchased and match it to Base Metal Group in Table 3.2 and filler metal in Table 3.3 to use a prequalified WPS.
It appears that you can join any group A material(3.2) with another group A material(3.2) and use filler metal from group A(3.3) and be considered prequalified. Join a group B material(3.2) to a group A material(3.2) you must use the filler metal(3.3) that corresponds to the lower strength of the two base metals to be considered prequalified.
"I think" you are OK as long as you match the filler metal correctly to the base metals, to be prequalified.
I too am looking at an old issue of D1.6.
John Wright