I don't think there is any problem using GMAW solid wire for SAW and getting good results. I was addressing the "without requalification" part of the question.
If the wire has dual certification from the manufacturer under both specifications (such as SFA 5.17/SFA 5.18), then it should not be a problem.
In ASME IX, QW-404.9 (essential)requires a change in the flux-wire combination to be requalified, with a few exceptions/conditions.
If notch toughness requirements apply, then QW-404.35 (supplementary essential) requires a change in flux/wire combination to be requalified.
If notch-toughness requirements do not apply and QW-404.9 is reviewed and it is determined that the change is within those exceptions/conditions, then QW-404.33 (non-essential) would permit the change without requalification. However, QW-404.5 must still be met where it requires the A-No. to be determined from the weld deposit analysis for SAW.
So, there are several opprtunities for "gotchas" to bite you in the butt.
Bottom line is the wire certs will need to be reviewed for each different wire classification and compared to the applicable code variables to determine if it is permitted or not.