I've had a number of responses to my last post on changing the way the US Senate is built, in particular pointing out that yes, it would be very difficult to make such a change given the constraint in Article V of the constitution:
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.
In other words, the 3/4 of states agreeing to the change would need to include at least every state that would be "deprived", i.e. have reduced representation. Since it's rather rare for any entity to give up power willingly, even for a good cause like equal representation, there would most likely need to be additional simultaneous changes adopted. Obviously this issue of upper house representation was a stumbling block in the original constitutional convention in Philadelphia all those years ago - but it should be noted that the states did give up power to the federal government in forming the union in the first place, so the process is not unprecedented.