I have a question since I have a really really old copy of ASME IX...
I assume you welded the original PQR with ASTM A633?
Is ASTM A633 a P-1 number material, or is it really an S-1, group 1 material?
Is ASTM A572 Gr50 a P-1 number material, or is it really an S-1, group 1 material?
If you welded the PQR with ASTM A633 Grade C, "S-1 Group 1 material" (assume grade C 65ksi min tensile)...
According to QW420, Welding Procedure Qualification with a base metal in one-S number grouping, qualifies for all other base materials in that same S number grouping.
If that works you will need a new WPS written for welding the ASTM 572 Gr50 against the original supporting PQR
However, IF you welded the original PQR with ASTM A633 Grade C and it was a 70ksi min tensile material, then you actually welded the PQR with a S-1, group 2 material, then the PQR will not support the new WPS.
There are two Grade C's listed for ASTM A633. 65ksi and 70ksi min tensile. 65ksi = group 1, 70ksi = group 2.