In addition to the up-turned nib (they call it a mini-fude), PenBBS has Fine and Medium nibs with traditional rounded tips. These can sometimes be bought on the pens, or separately in nib charms (nib units on acrylic key chains). They also sold some Extra Fine nib charms a little over a year ago, but they were as expensive as the pens from resellers by the time I started buying PenBBS pens. They're fairly cheap from PenBBS themselves, but they run out quickly and don't offer them often. Buying them from third-party vendors is much more expensive.
The nib charms have the advantage of being able to swap the entire nib unit.
For swapping nibs into the nib unit, the nib collar and feed take a liberal selection of number 6 nibs. Those mentioned above, plus Bock, Nemosine, Knox (some have had trouble due to their thickness, others love them in PenBBS), and others. Others have replaced the feed with FPR's ebonite feed and one of their ultra-flex nibs.
Removing the existing nib tends to be delicate. Others report often breaking the fins of the feed. I tend to cause a hairline fault in the nib collar that doesn't become apparent for a few weeks or months before it shatters entirely. Soaking the nib unit in hot/warm soapy water and going very slow seems to work for most. My preferred method is to avoid pulling the nib at all, and instead buying a parts pack from PenBBS, from which you can typically make five of your own nib units.
Your 456 model should have plenty of room in the cap to accommodate most size 6 nibs, but there's some worry over the 355 and 308 models (and possibly the 349?) that there's not enough headroom in the cap for most nibs which are 35mm long. I have been able to get my Bock B nib (my longest at about 35.1mm long) into both the 355 and 308, but the nib is jammed fairly deep into the collar, and some day it may join my ranks of trashed PenBBS nib collars.
Note that others have a lot of difficulty getting the PenBBS nib swapped into other number 6 pens, due to the squared back of the nib. I've haven't had problems fitting them in many pens, but I'm not that big a fan of the mini-fude, either, and have more than enough pens with it anyway. (It's not just a fairly small sweet spot, but it's a small sweet spot on more than one axis. I do have two or three of them that are exceptional, but that's out of many examples.) But it doesn't sound like you'll care much what happens to it when you're done with it.