RE: changing out the main gear..
Piece of cake.
1 - pop the 3 links that go from the control system to the outer star of the swashplate
2 - Loosen the mainshaft collar under the top mainshaft bearing.
3 - remove the bolt under the maingear/auto hub.
4 - pull the mainshaft/head/swashplate assembly straight up, out of the auto hub/maingear assembly.
5 - jiggle the maingear out the side
Voila.
NOTE: When removing the auto hub from the center of the gear, leave the auto bearing on the hardened steel hub and the lower e-clip in place so that when you re-install the auto hub into the new gear it will be in the proper orientation