Thanks for the replies, and sorry for not coming back earlier, but my router died and I've been pretty much offline.
The manual basically says that air pressure on the forks shouldn't be messed with, but it looks as if lots of people have done it and their bikes haven't exploded... Should have a look at it.
Anyhow, truth is that once I changed the rear preload, the bike feels much more stable and rebounds less on the front end (I was travelling at preload setting 2 with a pillion and allit boxes, so you can figure how much fun I had...), so I'm not really in any hurry to change anything now, and can look more into it. Will post if I end up changing springs and oil.
Apart from that, I guess I'll have to replace the broken sidestand switch, if just to keep the warranty, so the money is going into that first...