Which model of 40mm's do you have PEZ?
I can only talk about the ones I have experience with, the 1983 and 1988.
The early ones, e.g. 1983 have no circlip and yes you will need to dismantle. But it's a no brainer. Support the bike on a stand that raises the front wheel off the ground, put a container under the fork and crack the drain screw. You will need to undo the top air valve/nut to allow the oil to drain otherwise the airlock just keeps the oil in there. Leave it a little while. Do up the top fork nut/cap again so that the spring keeps pressure on the damper rod. Then calmly undo the bottom 13mm bolt on the bottom of the fork leg. Then the lower will slide straight off the fork leg. Make sense? My missus reckons I can't explain anything.. ;-)
Then find the biggest freaking screwdriver you have and prise the old crap seal out of the top of the fork lower. Sit down, put the lower between your legs, aim the business end away from you so you don't stab yourself in the head, jam the screwdriver under the old seal and lever away. Should pop out. Keep the old seal to help seat the new ones. Remember to oil the new seals lightly before assembly.
From memory.. (You have been warned!) the 88's are very similar to dismantle but a little more complex with a proportioning valve in the lowers. Only one seal and held in place with a circlip. Remove that circlip with a small screwdriver by slowly "walking" it out around the circlip groove.
Hope that helps?
FatBoyCrash