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I see two possible problems with switching classifications....
Ref AWS D1.1:1998 <---sorry the old copy is all I had handy.
1) Table 4.5 Filler Metal Item 3) change from one electrode or flux-electrode classification to any other electrode or flux-electrode classification.
2) M = mixed gas; C = 100% CO2 Table 4.5 Shielding Gas Item 19) A change in shielding gas to any other single gas or mixture of gas, or in the specified nominal percentage composition of a gas mixture, or to no gas.