there are at least several instances where the original constitution has become obsolete
ex: the 3/5 compromise - article 1, section 2, paragraph 3
the practice of carrying purses of gold and silver specie is archaic
even paper money is becoming outmoded by electronic transactions
yes, prohibited of states, but not prohibited of congress
more germane, issuing paper money was not absolutely forbidden by the constitution, as it is not enumerated in article 1, section 9 - limits on congress
strict constructionist philosophies have lead to absurd interpretations of the constitution, see the 'doctrine of absurdity'
the coinage act has been updated a few times over the last 200 plus years and the 'legal tender cases' settled this matter [see reply to 'gold standard', below]
long after 1792, president madison approved an issuance of 36 million dollars in paper money, to help pay for the war of 1812
it looks to me like his actions 25 years after the constitutional convention of 1787, as a more experienced statesman, speak more certainly than the words he wrote before
140 years ago, the legal tender cases began affirming the constitutionality of paper money
Knox v. Lee and Parker v. Davis in 1871, then Juilliard v. Greenman in 1884
In Knox v Lee, 79 U.S. 457 (1871), the Court ruled that paper money was not unconstitutional: "The Constitution nowhere declares that nothing shall be money unless made of metal." The Court argued that the Congress can manipulate the value of precious metals to the point where it can be rendered as inherently worthless as paper (the Congress could enact a law that says that 10-dollar silver coins weigh 400 grains in one year and 500 grains the next, effectively devaluing the silver). The Court even noted the arguments of the Framers against "emitting bills," but wrote that the Framers (1) could not anticipate all governmental needs and (2) allowed the Congress to do what was necessary and proper to carry out its powers. In this case, that includes printing paper money.
So, said the Court, even though paper money is not expressly permitted by the Constitution, it is also not expressly forbidden, and in spite of the extra-constitutional opinions of some of the Framers, the ability to print paper money is a necessary and proper power of the federal government.
http://www.usconstitution.net/constfaq_q154.html