Not sure I agree. Compress them enough times and you get plastic deformation, hence sagging. But on top of that you lose thickness in places due to corrosion which alters the rate.
If the spring you're replacing is original you will notice it sits higher when you're done.
Thickness changes (and coefficient of springiness changes) due to surface effects, yes. But you shouldn't get the plastic deformation with a properly defined spring. You shouldn't be anywhere near the elastic limit.
But as with most things that come in pairs, best to change together...
Nigel are these the best springs for adjustable osrav? is that what you are still running now? also on there softest setting what are they comparable to? These are what i think im going for but just need yo be certain that they can be adjusted to a nice comfortable ride for everyday use!