It appears that some Waterman nibs are interchangeable but, to be perfectly honest, this knowledge goes beyond my level of expertise.
I checked the web for some interesting/relating forum topics on this issue:
Apparently, only the nib and its feed need to be removed&replaced -- no extra parts.
Also, if you are looking just for the nib, you might want to consider asking more around the web (like the forum above) or look it up on eBay. Buying a whole pen for its nib might be unnecessary, and you could potentially save some money by purchasing only the one part you need. Others even suggest grinding the larger sized ball to a smaller diameter.
I hope you find this information helpful, and I wish you a great summer! :)