What's the difference between the different Bohnanza expansions? How do they change the game?
I can't answer your question in its fullness, as the only one of these expansions I have played is Bohnaparte, and it was once and a couple of years ago!  But I can tell you what my impressions of that were...

Bohnaparte constitutes a pretty drastic reworking of the Bohnanza game (to use a topical example, it's on a par with how Zooloretto expands on Coloretto).  Instead of a simple bean-farming game, you are now playing a Napoleonic wargame!  Albeit a simple, cartoonish one.  You lay out a map with areas of varying strategic import, and you now have to conquer it with your bean-troops, as well as growing beans in the usual way to maintain your economy.

At the time, I wasn't really a fan.  It didn't seem to me like a particularly deep wargame, or a particularly logical extension to the Bohnanza theme.  However, I do recall some nice mechanics: I believe the "valuable" beans (like Red) are weak in combat compared to the "common" beans (like Blue) - so it was a lot less clear which were the best beans: do you put a premium on combat or economic profit?  On the minus side, I remember the scoreline of our game being very close, and the result being basically decided on one player getting an extra turn over me due to the vagaries of the seating order.  That is probably true to the spirit of the original Bohnanza, I guess, but I remember being quite miffed at the time!

I guess if you play a whole lot of Bohnanza, to the point where you're getting jaded, Bohnaparte will certainly inject fresh interest into proceedings!  But I did think it was more of a novelty expansion than a must-purchase.  Unless your two favourite things in the world are the Napoleonic era and Bohnanza.  Then you should almost certainly buy it without delay!