Just replaced XT600 1VJ my fork springs with Hagon ones, left the forks attached to the bike throughout - removed the handlebars only. Didn't replace the fork seals though.
Drain the air from each fork, remove the cap on top (careful, they spring off). Draining the oil is easy, just open the little screw at the botton of each fork.Take the old springs out, there are two in each fork - little one at the top, replace the drain screws and fill with the recommended oil. Pop in the new springs (tight coils up) and tighten the caps, fill again with air. Easy.
The only problem I had was getting the caps on the forks again (the Hagon springs are a bit long than the originals). Had to get a mate to help, difficult but doable.
Lots more travel now, and the bike handles loads better. The old fork oil was black and thick, hadn't been changed in years (by prev owner), so not sure how much of the improvement is due to the fresh oil and how much the springs.
Also replaced the rear shock with a Hagon one, once again big difference.
Hope thats useful