[kokos]

Although I'm sure this has been answered in another thread, I will answer this as I did a spring change very recently.

The only question is: what is the oil viscosity and volume that the new springs' manufacturer suggest?

If it is the same as OEM (8w or 10w I think, and 143mm of air gap) then yes you can. Especially if you have a centre stand. Just put the bike on the centre stand (or support the swingarm if you dont have one) and raise the front end using a jack under the engine/skid plate. Remove handlebars, remove the fork caps, swap springs (if the new ones are progressive as well, make sure you place them in the same way round on both forks, regarding the tighter wound part - most will say upwards, doesn't really make any difference so long both forks are done the same way), put everything back together.

I think you may have to do a front end alignment after that (I did).

 

[greywolf]

Some oil will come out with the springs. You might want to measure the air space without springs and top off to 140mm or so with the front end raised to the stop.

 

[Tollster]

You will get a better ride if you take yout time and measure everything out. And the chance you will ever get back to do it again is unlikely. Remove, drain, measure, remount. You will be very happy you did it right the first time.
Maybe put it off till you have more time...its well worth it.

 

[PTRider]

You will want to back off the preload adjusters before removing the fork caps. They're soft aluminum, so be sure the screwdriver for the preload adjusters and the wrench for the hex on the caps fits just right to avoid damaging them. When replacing, you will need to press down hard to get the first thread to engage. Again, be careful so you don't bugger up the thread.

New springs might require changing the length of the spacer. 1" PVC pipe is easy to work with to make new spacers -- if you cut it too short, just toss and make another. A v-notch in the top to clear the cotter pin is a help.

Here's info about setting the sag which you'll need to do with new springs and likely different spacers.
Suspension Adjustment

The fork oil level is properly measured with the fork completely compressed, spring out, standing vertical. The book calls for 143 mm from the top, but some like 150 mm for more air cushion. You can probably pull out the old spring, let the oil drip back into the fork, and put the new spring in without making much difference in oil volume. Fork oil is Showa SS-8, which is 10 wt. with an actual viscosity of 35 centiStokes @ 40°C. This chart shows how other brands of fork oil of so-called 10 wt. actually compare. Some others range from 15 cSt to 47 cSt, an impossible range of variation and poor labeling.
http://www.peterverdonedesigns.com/files/suspension oils.pdf