It seems i should read the documentation (obviously) before posting
questions. The Box2d manual states: "Box2D supports the creation of
multiple worlds, but this is usually not necessesary or desireable."
So I guess I'll just clear the world for each new level. I can't mark
this question as answered for another seven hours. –
This part of the documentation signifies that it's not necessary or desirable to have two active instances of the World class at the same time.
When loading a new level you can just have the old world object garbage collected. You don't have to pull some fancy tricks to pass the world object from the old, now obsolete level object, to the newly loaded level object. Of course if your world object is static, or managed by something else than level. (Say a LevelManager or something) then clearing the world object and then adding all the new stuff to it is just as good (or maybe even better), but both approaches should not bring any noticeable changes.
Btw, I'm not sure how far you are with learning Farseer, but I do know that it's quite hard to find tutorials, a user called RabidLion updated my Farseer tutorial to Farseer 3 and added quite a few extra things so it might be worth looking at: http://roy-t.nl/index.php/2011/01/12/xna-farseer-platform-physics-tutorial-updated-to-farseer-3/