Well, from my Boscom spring swapping experiments I still maintain that springs/hammers have not changed through the Model M ages, from IBM to Lexmark to Unicomp.
Show Image
(http://farm2.static.flickr.com/1275/5182969903_a864a1a856_b.jpg)
But I think a lot of factors could affect the springs. Age, use, debris/deposits on the coils, etc. If the hammers are identical, I don't see that age / use would affect them much unless they are undergoing some sort of chemical change/deterioration. Perhaps if they were used very heavily they're suffering from getting worn out? Or what's this about the wrong seating making them feel odd? Is it possible that their original seating is being changed over time due to use, etc? Would reseating the springs properly bring the keys back to their original feel? And is it possible that keyboards in their original state had some differences in how the springs were seated and thus a slightly different feel even when new?