Past getting the spring out, I think you just dump the oil, refill, pump the damper to get the air out, set the oil level, and go. There is a trick to setting the rebound clickers but it has been a while since I worked on XR400s. I think what I did was turn the rebound out to full soft, then turned it in the number of clicks it is supposed to have for total adjustment, then two clicks more for safe measure. You then turn the cap down onto the rod until it lightly seats. This should insure that you have the full range of rebound adjustment. If you install full soft, the you won't have any adjustment and from what I remember, if you turn the adjuster in too far with teh cap off, the rebound adjuster will start leaking (had to replace one myself).
As far as further disassembly of the forks, you will probably need a damper holder and/or an impact wrench to get the center nut out of the bottom and that will allow you to take the damper completely out of the bottom tube. I don't think this is necessary for regular fluid changes and cleaning.
I have a Clymer manual for the XR400 and I followed it step by step when disassembling my forks, changing oil, and reassembling. The pictures are helpful. I sold the bike and would be glad to let the book go fairly cheap..email me if you are interested.