I don't remember how changes in 300 series filler metals were covered. If my memory is any good, Mil STD 278 is the code/standard for fabrication and ot qualification. I think that the section in that standard which refers qualification of welders and procedures indicates that procedures qualified in accordance with Mil STD 248 or ASME SEC IX may be used. If the procedure is qualified and filler metal classification is a non-essential variable, yes you could substitute as far as the qualification goes however that does not mean its a good idea. Many people say that thats an "UPGRADE". I suggest apporval by the engineering authority or customer if you are deviating from what your original WPS stated.

If I had the codes, I could help you but it's been 14 years since I opened a Mil Std 278.

Have a good day

G Austin
http://www.weldinginspectionsvcs.com