Legal Realism
The preferred term for "legal instrumentalism," which is forbidden to be used.
Legal realism is the philosophy that the law, and, more specifically, its interpretation, must serve (i.e., be instrumental to the cause of) human progress. As such, it is thought to be malleable in an organic sense, provided it is reposed in capable hands. In that sense, it doesn't actually matter what the incipient principles of a legal framework are, as long as they can be properly shaped by progressive thinking. Thus the U.S. Constitution is kept around for sentimental reasons, though any serious discussion of its founding principles, like the separation of powers, enumerated powers or "the laws of nature and nature's God" is either forbidden or severely restricted, and the preamble's expression "promote the general welfare" is regarded as bestowing plenipotentiary powers on government.
The primary approach to altering the Constitution via judicial fiat is to hand down an interpretation, however specious, and subsequently appeal to the so-called doctrine of stare decisis which originally arose in the context of Common Law, but which was incorporated, apparently without serious reflection regarding the consequences, into Constitutional jurisprudence. Thus a modern or post-modern interpretation of a paragraph or clause in the Constitution can be utterly at variance with the common understanding of both the framers and those who voted to ratify the Constitution, and only "originalists" can find any reason to object.
At this point, the astute reader may be reminded of the song, "Tradition"! in Fiddler on the Roof.