"rlwainwright, have you never heard a difference in sound between a new product, what ever it might be, and the same product with some time on it? if not, you must be joking right !!"
Indeed, I have heard differences in aged components over new ones. But they were manifested in speakers, cartridges, and, to a small degree electronics. The first two are easy to explain - you have devices that are converting one form of energy into another and doing so using mechanical processes.
Speaker surrounds will loosen up a bit and achieve a "set". The same can be said for the cantilever of a cartridge and its rubber surround. I can also envision the wires and various circuits of a pc board having some of their properties altered slightly via the heating and cooling cycles that take place when the unit is first put into service.
However, I cannot see ANY reason that a binding post would undergo some kind of transformation that would be measurable, let alone audible. It is not being subjected to temperature extremes, it has no moving parts, and it is not being subjected to high voltages or current. It's a lump of metal, plain and simple.
My best guess as to why your system sounds different is because of the processes involved in effecting the upgrade: the unit was removed from its usual location, speaker cables were removed from the original binding posts, the unit was opened up, and fiddled with. Tubes got jostled. Wires were heated up in order to remove them from the original posts. Wires were again heated up when attaching them to the new posts. The unit was then re-assembled and the speaker cables were re-connected.
Any of these could cause some small change in the sound. The breaking-in of the binding posts was most certainly not the culprit, there is nothing there to break-in...