Hi Sean,
AWS D1.6:1999
Para. 4.2.1 Changes to Essential Variables.
Any change to variables set forth in Table 4.1 shall be considered essential changes and shall require the qualification of a new WPS.
Table 4.1 WPS Qualification Variables says under 7.1
A change from a single shielding gas to any other single shielding gas or to a mixture of shielding gases, or a change in specified percentage composition of shielding gas mixture, or the omission of shielding gas.........for processes: GTAW,GMAW,FCAW, and PAW
I think this is the info you was looking for,
John Wright