So I'm going to lay out the way the current problem looks to me at the moment here.
New positions in DF2010 cannot simply appear, unless they meet their requirements at the start of the game. They need to be appointed or elected. So the only way to replace the highest-authority position at a site is to make it appointed by a civilization-level position or make it elected. The latter is obviously incorrect. The former would be ideal, but is tied to land holder weirdness - in particular, LAND_HOLDER_TRIGGER does not appear to function correctly. It seems to consider the conditions for LAND_HOLDER:1 to be met as soon as the fortress is founded, regardless of the actual requirements for that trigger.
You can however make all of the positions stem from one position (expedition leader for example) and simply have the rest of them upgrade from the appropriate. You do not need an elected or landholder tag if you have all of the appropriate levels stem from the expedition leader which of course you get at the very begining. A good example is the CoG position which upgrades from sheriff, because it is derived from the expedition leader it is able to exist.
If you want you can take all of the responsibilities away from the EL and just leave him in as a figurehead type position. I made my own caste system which stems solely from the EL and it works perfectly.