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.
Hamish