Most people think of the typical Bb clarinet when picturing a clarinet. However, there are several styles or types of clarinet. http://musiced.about.com/od/beginnersguide/a/clarinettypes.htm The most commonly found in bands are the Bb, alto, and bass clarinets. How hard is it to move from one type of clarinet to another? Not hard at all!
The great thing about playing different keys or versions of the same instrument is that the fingerings for the different notes are basically the same, with minor differences. The main playing distinction between the different clarinets, besides the obvious size differences and pitch ranges, is the size of the mouth piece. This can take some getting used to. However, once you have been practicing regularly, your embouchure (the muscles around your mouth) adjust.
When teaching private clarinet lessons in Eldersburg, I always stress the importance of practicing on a regular basis. Playing different instruments at the same time, makes this even more crucial. For example, I have one student who plays Bb clarinet and bass clarinet. Sometimes, due to availability of the bass clarinet, he practices his bass clarinet music on his Bb clarinet. Therefore, while the fingerings are basically the same, it's important for him to have practice time on both mouthpieces, since the size difference is significant. While initially challenging, the transition is not that difficult.
Back to the different types of clarinets, as mentioned earlier, the most commonly found clarinets in school bands are the Bb, alto, and bass clarinet. The Bb clarinet has the smallest mouthpiece of these three. The alto clarinet mouthpiece is very similar in size to an alto saxophone mouthpiece (which is convenient for those who play both instruments), and the bass clarinet, obviously, has the biggest mouthpiece. Going from a small mouthpiece to a considerably larger one can be challenging, but, again, with consistent practice, you can do it.
Instrument care and accessories for the different types of clarinets is basically the same. You need a proper reed, cleaning swab, cork grease, and, in the case of the alto and bass clarinets, a neck strap.
If you are a clarinet player and you have the opportunity, I encourage you to try one of the other types of clarinets to expand your playing experiences. It's fun!!