L.W.
One of my good friends slowly transitioned her daughter from breastmilk to rice milk (she's vegetarian) pretty easily. She added the rice milk to the bottles of breastmilk. She started off with a small ratio of rice milk to breast milk (like one ounce of rice milk to 5-6 oz of breastmilk) and over time she upped the rice and lowered the breast.
Her baby didn't seem to notice the transition (it took about a month in total) and her daughter laps up the rice milk like it's the best thing since sliced bread!
Good luck.