Vincent,
In AWS A5.18 2005 (thats the revision I have)
ER70S-6 and ER70S-G are two different classifications.
While both are covered in A518 they are still different classifications.
Furthermore ER70S-G is a special catatory with an A5.18 footnote reading as follows:
"Chemical requirments are not specified but there shall be no intentional addition of Ni, Cr, Mo, or V. Compositon shall be reported. Requirements ar those agreed to by the purchaser and the supplier."
The ER70S-G has a compositon that only you as the purchaser (and PQR developer) and the vendor that sold it to you are aware of. So if you are changing it, good engineering principles would suggest that you requialify to ensure you are getting the mechanical and performance properties you desire.