In an 1816 letter to a Virginia lawyer, Thomas Jefferson said this, which is enshrined on the Jefferson Memorial (panel #4):
I am not an advocate for frequent changes in laws and constitutions, but laws and institutions must go hand in hand with the progress of the human mind. As that becomes more developed, more enlightened, as new discoveries are made, new truths discovered and manners and opinions change, with the change of circumstances, institutions must advance also to keep pace with the times. We might as well require a man to wear still the coat which fitted him when a boy as civilized society to remain ever under the regimen of their barbarous ancestors.
There's more to it (text here; image here), but what other examples are there of Founding Father's being open to or advocating the Constitution as a "living" document, which can and should change over time?
There is ample evidence in the text itself. Article V directly states:
The Congress, whenever two thirds of both houses shall deem it necessary, shall propose amendments to this Constitution, or, on the application of the legislatures of two thirds of the several states, shall call a convention for proposing amendments, which, in either case, shall be valid to all intents and purposes, as part of this Constitution, when ratified by the legislatures of three fourths of the several states, or by conventions in three fourths thereof, as the one or the other mode of ratification may be proposed by the Congress; provided that no amendment which may be made prior to the year one thousand eight hundred and eight shall in any manner affect the first and fourth clauses in the ninth section of the first article; and that no state, without its consent, shall be deprived of its equal suffrage in the Senate.
By including a method for amending the document, it's clear that the framers recognized that there would be a time in which subsequent generations would deem certain changes necessary. By making the process for amending the Constitution difficult, the framers are essentially taking the exact same position as that which Jefferson is taking - that changes should be possible, but not so easy that they are frequent.