In the baking section of our grocery store, I bought a powdered orange that can be used in baking etc. It has no artificil sweetners, nor hardly any sugar, and was great mixed into water. My daughter also likes "limaide" lime juice in her water - especially when she gets to squeeze it. (She is 3 1/2).
We had the same problem with milk/chocolate milk, because she does NOT get juice, except at birthday parties and sometimes at restaraunts. We made a felt chart on the wall, and I cut out 2 white milks and 1 chocolate milk (dr. said to try not to let her have much more than 25-30 ounces of dairy, and she eats a lot of cheese and yogurt). There are also lots of blue waters
Every time she makes her choice, I move that cup to the other side, so she knows it is now off the list. She now knows her limits, and makes the concious decision to have a water with snack because she wants a milk with dinner, or vice versa. We aren't fiting because "she is in control".
Other than that, I would just start watering down the juice slowly more and more over time, and eventually give her no more than 1/4 juice, the rest water. Since she is a juiceaholic I would do it in the bottle, since at that age she is definitely smart enough to see what you are doing if you don't fill the glass, and then walk to the water!
Good luck.
PLEASE cut out the juice during the night immediately! If milk is bad for little teeth at night, I can just imagine juice! You may have 2-3 bad nights, but it is worth it I hope!
Many pediatricians recommend that juice only be drunk in the kitchen when it is given, since the most damage is from sipping slowly over time on the teeth. Perhaps you could try a rule, she can drink her juice standing a the sink, but she can't walk around like that.
Additionally, we have a grandparent with false teeth. We explained to our 3 year old that you lose your teeth if you don't take care of them, which quickly fixed the teeth brushing fight.
She is old enough to explain what could happen to her teeth!