Is there a preferred time of the year to repot Nepenthes, or is one time as good as another? Should the original medium be teased away from the roots and discarded, or moved with the plant into the larger pot and into the new mix? The plants in question are in small 2- or 3-inch pots straight from Lowes. I'm concerned that the very small pots make the plants more vulnerable to watering mishaps.
Theoretically, Nepenthes can be repotted at any time because, unlike other carnivorous plants, they don't have a dormancy period. However, given that most growers are not in the tropics, in practice it is different. It is best to repot Nepenthes in spring and summer, when the weather is warm and the days are longer. This gives them better conditions to recover with. However, if you grow your Neps in a purely artificial environment, it probably doesn't matter.
Many Neps don't like thier roots being disturbed, although most of the beginner species and hybrids are very tough. I generally gently shake as much of the old medium as I can from the roots without disturbing or breaking them too much. If the medium won't break up, just place the entire root ball with old medium into new medium - it's better to leave things be than hack into it. That having been said, I have a hybrid that I didn't really want to I hacked it back, chopped off most of the roots and repotted. It lived, and is now growing well. So sometimes they can take brutal treatment. I wouldn't try it with bellii, bicalcarata or lowii - disturb the roots of those types of species and you have a high probability of the plant simply dying.
I agree with Hamish. I don't like transplanting in winter. Especially if the plants are outdoors like I grow them. I grow in sphagnum so I repot leaving the original rootball intact. Before I repot, I make a mold in the new bigger pot with wet sphagnum using an old pot the original plant is grown in. Then I scoop the whole thing out of one pot with my hand (plant and soil) and fit it right into the mold in the new pot. This way the roots are never disturbed and it makes a perfect fit. Another reason why I like sphagnum over some other media because it doesn't fall apart during repotting.
The only time I shake off the moss when repotting is if a Nep is in trouble. If it is dying for some reason, I will discard all the media and repot with fresh stuff to try and save the plant. Other than that, I use the my old media into a bigger pot. If the plant is doing great, why mess with it.
Nepenthes Around the House