Depends on what you are doing. Going from one fixed A1/A2 style stock to another is as simple as removing a screw. Going from a fixed to a collapsible is more involved, but still doable. Rock River has the castle nut wrenches for about $4 (or used to at least).
This link shows the process:
http://ar15.com/forums/topic.html?b=3&f=4&t=226782[This message has been edited by Log (edited 12/23/2007 11:50a).]