I'm thinking about making kefir. Probably water kefir since I think it might taste better than milk kefir and I like the idea of having something a little fizzy to drink. But if I decide to make milk kefir later can I use the water kefir grains? Can I go back and forth between using them? What's the difference between them anyhow?
No, kefir grains don't like to be changed the kind of milk they use too often. They like it even less going from water to milk and back again. They won't thrive, they might even start dying out. What you need to do is buy your kefir grains (possibly separately for water and milk) and make two different batches daily. When it comes to milk, go for sheep/goat btw, not for cow milk. These have a much more tolerable casein. Ferment for 24 hours for less lactose. Also, make sure your strainer/utensils are made of plastic (kefir grains don't like metals).
No, they are different strains. Some people can get a culture or two out of the grains in non preferred substances, but the grains almost always die.
That said, once you have used the grains for a while, they will multiply, so you can use your extras as testing grains.
To the OP -if you haven't found it already, here's a useful website http://users.sa.chariot.net.au/~dna/kefirpage.html
I've been doing water kefir for a couple of months and some of the tips there have worked pretty well (interesting info also)