I'm not sure I understand this talk of "moral frames of reference" vs simply "values".

By analogy, take a complicated program as "frame of reference", and state of knowledge about what it outputs "current values". As you learn more, "current values" change, but frame of reference, defining the subject matter, stays the same and determines the direction of discovering more precise "current values".

Note that the exact output may well be unknowable in its explicit form, but "frame of reference" says precisely what it is. Compare with infinite mathematical structures that can never be seen "explicitly", but with the laws of correct reasoning about them perfectly defined.