Will shortening the shock require stiffer springs?
Stock sag is 1/4 i 1/3 of the suspension travel. So if your lowering it id just set the sag at 1/41/3 of that which will be more like 7090 instead of 90110.
And for your question kinda but no. Sag can be ajusted by preload, it doesnt have much to do with the weight of the spring. You can make a kx85 or a kx65 spring have the same sag as kx250 spring if you put a ton of preload on it
I lowered my DRZ by 1 1/2", never could get it to ride right. I got a stiffer spring and it's a lot better now. You shouldn't need to go up a lot if the stock spring rate is correct for you. At full length, stock was good for me, but I went from the stock 5.3 to a 5.5Kg for the reduced travel. Definitely stiffer springs in the forks though, I went from .44 to .47Kg. (I weigh 180lbs and ride rough terrain)
If you really want to know, here is one way to calculate the shock spring rate when lowering a bike. It is pretty generic and will need to be tweaked for each application.
Normally when we give this information out, well you know the saying. But this time we'll let you live.
1. the formula
spring rate = ((wt bike + rider) x 1.40 x flr) / shaft travel
where flr = final leverage ratio
flr = wheel travel / shaft travel
2. wt bike + rider
wt bike = 240
wt rider = 180
wt bike + rider = 420
wt bike in kg = 191
3. flr at original travel
wheel = 305mm
shaft = 125mm
flr = 305 / 125 = 2.44
4. flr at lower travel
lower the bike 1.0 inch (25mm)
some guess work is needed. we need to know the initial lev ratio.
we will estimate the initial lev ratio to be 3.00
here we go:
wheel = 305mm  25mm = 280mm
you might have to think about this for a minute:
shaft = 125  8 = 117mm
shaft is reduced by 8mm because: 25 / 3 = 8
(25 wheel reduction / initial lev ratio)
therefore, the lowered numbers are:
wheel = 280mm
shaft = 117mm
flr = 267 / 117 = 2.39
5. so we have the flr for both the stock seat height and the lowered seat height:
stock seat ht = 2.44 flr
lowered seat ht = 2.39 flr
6. plugging in the numbers
stock spring rate = (191 x 1.40 x 2.44) / 1.25 = 5.22
lowered spring rate = (191 x 1.40 x 2.39) / 1.17 = 5.47
Or: if you lower the bike and try to use the same spring, simple reasoning tells you that the stock setup had a 5.2 spring that was compressing 125mm at full bottom. now you have the same spring trying to control the same bottoming force and it only compresses 117mm. therefore, with the lowered setup, the shock bottoms before it reaches the same tension on the spring.

Thanks! My next question would be how to figure out the new spring rate but I think you covered that too. FWIW I'm right at the upper end in weight for the stock spring. I think the way to tell is to set the sag right now at stock + 2" which would be the same preload when shortened which I know right now will be way too soft.
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I want to do this to reduce seat height, so I plan to run the same sag as stock. The way I see it is shortening the shock (I'm looking at 1.52" overall rear travel reduction) is bascially the same effect as increasing sag (but the spacer takes out the the extra sag). Therefore the spring would then be way to soft, same as running 2" more than the stock sag. Is this right?