Look over the essential variables(Table 5.3)....it appears that they allow a change in electrode diameter for SAW of one size...not sure where your other variables fall, but make sure they are within Table 5.3...They do have a WPQR for the original WPS...right?
Is this the same electrode mfg as the smaller size wire for which the original WPS and WPQR were written?...(see Table 5.1)