I'd like to do this only once so:
Replacing KYB's with Bil's, not changing springs (100 Lb, about 12 years (15-20K miles) old). As I visualize the setup for the rear, If I keep the distance from the lower spring perch to the lower mounting bushing the same, the rear ride height will stay the same as before, though possibly a bit higher because of the apparently higher pressure in the Bil's.

Thanks,
Harvey Hoover

sounds like you've got it right, but I wouldn't expect it to be much (if any) higher.

I love the mystique surrounding springs. The first job I ever had at 17 years old was making springs. Automotive coil springs are designed with stress limits appropriate for the deflections involved. You will not get "fade" over time with these springs. They may fail in catastrophic mode, IE: BREAK, but they are not likely to fade over time. So don't spend money for springs that are of the same rate as your existing springs unless they are BROKEN. Shocks will wear and their performance will degrade over time, but springs either resist deflection with a predictable load or they break.