Either the base metal specification is listed by an AWS structural welding code or it isn't. If it is listed, it is prequalified. If it isn't listed, it is not prequalified and can only be used if the procedure has been qualified for the specific material used in production.
The contractor must use the material specified by the approved contract (design) drawings unless the "Owner's" Engineer (as defined by the applicable welding code) approves the substitution. That allowance is for that project only and does not extend to other project for different Owners.
CVN is only required when it is specified by the project specification or design. The Owner's Engineer is required to specify the test temperature and the required test results.
In this case I would ask the customer, e.g., the Owner, if they are allowing the material substitution, assuming the Owner originally specified a material listed by the structural welding code. Just because the materials may have similar chemistries, it doesn't mean they have the same mechanical properties. That's why AWS structural welding codes do not include any AISI steels which are specified by chemistry only.
Al