Hi John!
I don't have D1.1 2008 on hand but, in the 2002 edition i do remember that 4.7 Essential Variables:
Changes beyond the limitations of PQR essential variables for the SMAW, SAW, GMAW, GTAW, and FCAW processes shown in table 4.5 and table 4.6 (Which is underlined to referring that if the PQR required CVN Testing then table 4.6 was applicable instead of Table 4.5) when CVN testing is specified, shall require requalification of the WPS (See 4.1.1.3) which only refers to CVN Requirements so if you had to perform CVN testing then, this would be applicable to you: 4.1.1.3 CVN Test Requirements.
When required by contract drawings or specifications, CVN test shall be included in the WPS qualification (I know you didn't mention CVN testing but, I'm including this clause for educational purposes). The CVN tests, requirements, and procedure shall be in conformance with the provisions of Annex III, or as specified in the contract documents.
So, if the WPS didn't have CVN requirements, then we could go to table 4.5 which says in the section for Shielding Gas:
19) A change in shielding gas from a single 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. And if you go across that line 19), you will see under the GMAW & FCAW processes that there is an "X" in the corresponding box which tells you whether or not the PQR Essential Variables Changes Require WPS Requalification and in your case you did just that... Provided that you also checked line 21) "A change in shielding gas not covered in:" Which is referring to A5.18, or A5.28 in GMAW and A5.20, or A5.29 for FCAW
So now you want to know if the welders are required to requalify to the newly requalified WPS due to the change in the composition of the shielding gas from an 80/20 mix ratio to a 90/10 mix ratio of Argon/CO2...
Well, according to 4.22 "Essential Variables" in Part C Performance Qualification it states: "Changes beyond the limitation of essential variables for welders, welding operators, or tack welders shown in table 4.11 shall require requalification."
Okay then, let's go to table 4.11: It shows me eight Essential Variable Changes to WPQR Requiring Requalification. So I go down the list and, line 1) isn't applicable since you didn't change the process from the original WPS... Line 2) Isn't applicable because you're not using the SMAW process... Then I read line 3) which states: "To an electrode and shielding medium combination not approved by an AWS A5 document." So, that tells me that you need to check with the A5.18, or A5.28 for GMAW, and A5.20, or A5.29 for FCAW as referenced in Table 4.5 in item number 21 (A change to a shielding gas not covered in:) found in page 135 in the D1.1 2002 code. (I don't remember where it is referenced in the 2008 version)
So I go down the rest of the lines, and find nothing else that may relate to your situation which to me means that only line 3) could be applicable and a reason to requalify the welders provided that the shielding gas mix is NOT approved in A5.18, or in A5.28 for GMAW and A5.20, or A5.29 for FCAW.
So unless the shielding gas is not covered in A5.18, or A5.28(GMAW), and A5.20, or A5.29(FCAW), line 3) in table 4.11 does not apply to your situation but, if it does then requalification is required according to Table 4.11. So, if this is the same in AWS D1.1, D1.1M 2008 then, like I said before unless the gas mix is NOT approved in AWS A5.18, or A5.28(GMAW) and, A5.20, or A5.29(FCAW) the requirement for requalification of the welders is NOT necessary... Clear as mud John !!! :) :) :) Hopefully this will lead you in the right direction. :)
Respectfully,
Henry