Paul, I've not done it on the Versys yet, but it usually is a fairly straightforward matter of opening up the top caps and then opening a drain at the bottom near the axle.
As for the amount of replacement oil, it's difficult to say for sure because it has everything to do with how much of the existing oil you're able to get out and how much remaining volume is left to fill. It's normally more a measurement that you make as you're doing verses knowing exactly how much you will put in going into the job - kind of like changing the engine oil. You can have a pretty good idea how much you'll put in, but it might be a little more or a little less.
On the KLR, we used a trick where we would take the sprayer and tube out of a bottle of window cleaner. We placed the tube down into the top of the fork tube at the depth of void we wanted to leave above the oil. We then marked the point at which the spray tube exited the fork tube. You can then fill the fork tube up past the level you want, and use the sprayer tube to pump out the amount you don't need down to the end of the spray tube. Since you marked the spray tube, you can then move over to the other fork tube and set the level to the exact same depth.
With all of that said, have you made full use of the suspension adjustments available on the front forks. I have yet to mess with my front suspension on the Versys, but I'm pretty sure you can adjust preload and dampening with the adjustment screws on the right hand fork. Obviously that will only take you so far if you need a different weight of fork oil, but I have not heard of anyone maxing it out in either direction, so I would think the factory weight is okay.
Anyway, hope some of this is helpful.