I'm not convinced that the rates of your springs are as marked. The 9 coil ones calculate to about 900lb (907lb) and the 10 coils ones to about 800lb (816lb). I will check with my spring man during the week and come back to you on that...
However, assuming your springs are marked correctly:
If your springs are UK made, the wire used is probably 0.5000" diameter (which is 12.7mm) without rust/paint etc as that is SWG -7 and most springs are made from standard gauge wire rather than metric or special sizes. They could be made from 13mm wire, but it is not very likely.
http://www.skegsprings.co.uk/technical/size.asp
The next standard wire size down is 0.464" which is SWG -6. Metric equivalent is 11.7856mm.
To answer your question about changing wire size, I'm not sure what the 'standard' metric sizes are, but to reduce your 600lb springs to 550 lbs springs, the wire size would need to drop from 12.7mm to 12.435mm - which is only a touch more than 10 thou! To reduce the 550lb springs to 500lb spring, the wire diameter would have to drop from 12.7mm to 12.465mm - about 9 thou. It is because these small diameter changes have such a major impact on spring rate that accurate measurements are required when calculating rates.
If your springs were made with SWG -6 (0.464") - the next standard wire size down, the 9 coil springs would have a rate of 440lb and the 10 coil springs would have a rate of 396lb.
However, using this gauge of wire, 8 coils would give a 495lb spring, 7 would give a 565lb spring and 6 would make a 660lb spring.
The usual way to design softer springs is a combination of wire diameter and the number of active coils. As you can see, using 12.7mm/0.5" wire, adding a coil to the 9 coil springs gives a 60lb drop, adding another coil to the 10 col springs would give a 49 lb drop down to 491lbs. One more coil (12) would make a 450 lb spring (a drop of 40lbs) - as you can see it doesn't appear linear, but it is if you start with only one coil: One coil would give a rate of 5400lb. - therefore 10 being 540lb should make sense and you can see that 20 coils would make a 270lb spring.