If flight tests turn up issues that require design adjustments, any already built tankers will have to be retrofitted, which is much slower and more costly than doing modifications during the initial assembly process................
Indeed. And this has bit Boeing badly on the 787 program. Of the fleet of flight test aircraft, only one has been successfully refurbished for delivery. And that took so long and was so expensive that no others have been refurbished. And the next dozen or more airframes after that (the "terrible teens"') are sitting in Everett without engines and (at least so far) without prospective buyers because they don't know what it is going to take to certify them for delivery. And even if change incorp and certification is accomplished, they will never have the performance of the later airplanes and because of their uniqueness, the buyers will always have support/maintenance issues with them.