It's not a huge ball-ache, but on a ball-ache scale of 1-10 it ranks up as an 7 = achy swollen nads.
Biggest pain is getting the bearings to seat properly when ya rebuild them - if you've got a real thin jewellers screwdriver, you can use it to push the bearings back in when they unseat (when you're putting the arms back on - you'll see what I mean when you try it).
Couple of other tips - loadsa grease when you're rebuilding them to keep the bearings in position, take them apart carefully over a bowl, and count the bearings as they come out. Use the jewellers screwdriver as well to undo the notched bearing bolt-thing. Don't do anything recreational when you're doing 'em n'all as you'll lose the will to live and have to have a lay-down
There's a rough look at doing the swap over at the museum - done it a few times, and the bugger will still work even if you lose a couple of bearings (or even if you've bought one where someones lost a load of the bearings but didn't tell you....
)
http://bmxmuseum.com/forums/viewtopic.php?id=386250
