have a look at Dom's Kefir page:
http://users.sa.chariot.net.au/~dna/Makekefir.html
and scan on lactose

the kefir grains will ferment the lactose into lactic acid (and a small bit of ethanol). How much lactose (milk sugar) remains is largely a function of how long you let it go - how sour you let it get.

if you are concerned about CHO content remaining, you might want to just try a test: if your label says 22g CHO -- drink milk serving with 22g CHO and test your BG. Later, under same conditions, drink kefir serving with 22g CHO and see if it effects your BG differently.

if you are concerned about lactose because you are lactose intolerant - you'll have to try a test of kefir and see how it goes.

sometimes commercial kefir is flavored and has added sugars - obviously you don't want brands like that.