Ideally, when it is dormant, in winter. But as long as you prepare a larger pot first and move the whole of its contents, as one unit, you can do it anytime. The idea is to minimize root disturbance and when it's asleep or carefully removed, it doesn't notice anything.

From what I've read... DONT DO IT!

From what I've had happen when I tried it... BE CAREFULL LOL. When I tried transplanting my first VFT I damaged his roots. Belive me thats way easy to do. I lost my first fly trap because of that. The fly trap roots are more fragile then you might think. Seriously.

If you're intent on doing it... Yes, durring winter domancy is best. If it doesn't go dormant this winter, yes you can still do it just make sure you move all of it, as carefully and without disturbance as possible to a pot of larger size.

The medium I use is half spagnum peat (not the green stuff) half washed sand box sand. Seems to keep them happy.