It helps that you take one of the donor cases all the way apart and don't need to put it together again - the donor for the planetary reduction gears that goes in the new case. So you get a little practice on the way they come apart before you tackle the second box that needs to come even farther apart (removing the input shaft) and reassembling.
It's not all that hard, though. There's never a part where little spring loaded parts flew out or anything.
Some basic tips:
1) When removing the rear case - undo the output flange, and then remove the shifter fork detent balls (two allen headed covers on either side of the shift rod ends). Then the cover can come off without disturbing anything inside - the forks stay with the gears, the front drive stays put, just the cover comes off.
2) Then pull out the output gear cluster with the shift forks in place as a unit, that way the shift collars never pop out and you don't have to mess with the incredibly fiddly spring loaded syncro dogs.
