There's a sticky in the "Setup and Repair" forum that points to this site (among others):
http://www.garywillis.com/pages/bass...tupmanual.html
He explains that buzzing in only the first five or so frets suggests a need for more relief in the neck, whereas buzzing in other places requires different solutions.
It isn't at all uncommon for new strings, especially when of a very different type, to require some setup modifications. With some strings (i.e., with very high tension) you can get away with a very flat neck, whereas with others you need more relief to avoid buzz.