Since the applicable code is ASME, the one problem that might apply here is a change in the A number.
If I remember correctly (and it was a long time ago that I worked with 309/310), it was either 309 or 310 alloy that had an A number of 9. In reading the applicable paragraph in the code, it allows that a sample of the weld deposit taken from the PQR coupon can be used to determine the P number. The A number may change depending on the dilution obtained in welding the PQR versus the production welds, i.e., grooves versus fillets.
The alternative would be the A number determined as per the filler metal specification.
One consideration, and I haven't looked at it, is what is the anticipated ferrite number? Can it change due to differences in dilution between the single pass groove weld, the fillet weld, and the multiple pass groove weld? Will hot cracking be a potential problem? I guess that's where it may pay off to look at the WRC diagram ( DeLong diagram) to calculate the ferrite number.
Best regards - Al