ok. don't know if this is right for the year/model you have, and i haven't pulled a set of them apart myself, but here goes.....
aparrantly they run a combination of elastomer stack and spring.
when you undo the top cap (carefully cause it's under load) you should find that the spring/elastomer will just pop right up. you will then be able to see if it is connected to the top in any way, or if the preloader is simply a block that screws down onto the top of the stack. if it's attached you will have to undo the thread off the cap, and relase the circlip inside. the older model apparantly is NOT attached.
when you have the top cap off you will be able to remove the spring/elastomer stack by hand. geoff reckons the best place to put a spacer is at the bottom of the elastomers, between the spring spacer, and the last rubber. start with about 5-7 mm and work up from there.
now comes the interesting bit.....
older model judy tt coils are (i have been told) just about 5mm longer than the whole elastomer stack, and about 20% stiffer. so if you have/know of a set of old judys (or pretty much anything else i would guess, but he only mentioned judys) you can replace the elastomers completly, and get a better feel to boot.
i repeat that i do not have any first hand knowledge of this myself, but the guy i was talking to has been racing his for a cpl of seasons back home and reckons this was the way to go.
and in any case there should be no trouble in removing the top cap to have a look anyway, so go for it.