I don't know what the official committee interpretation would be, but I would argue that if the essential variables for both PQRs are the same (except for base metal, of course) that there would be no metallurgical reason why the same welding procedure used to join the two different grades together would ever be a problem. There is nothing magical about the admixture of the two different grades of steel pipe within the weld that would cause significant or unacceptable change in weld quality or properties.

It is my understanding that a PQR supporting X70 to X70, also qualifies for welding X70 to lesser grades of material.

See 1104, 19th edition, paragraph 5.4.2.2, the first sentence.

Charles Hall