This response is based on the information provided.  We suggest you discuss with your contracting team, program manager and/or legal department as appropriate. 

There is too little information provided to give a definitive response. 

As always, "The answer is in the contract."  The language of the schedule (Section B, CLIN structure) supersedes the language of the PWS.  You have an inconsistency in the language as presented to us. 

If you believe the benefit over the life of this contract is significant enough to make this change, then you need to identify this discrepancy and negotiate the change concluding with a supplemental agreement.  Before you do, check to see if that there no other discrepancies written to our advantage which the contractor would want to negotiate.  The contractor has a right to assert this modification will cost it money and that it deserves some consideration.  But they also have to prove they are going to incur additional costs, i.e., the fact that we are paying higher then we ought and no longer choose to do so is not satisfactory justification for some price increase.