you have a lot of responses, but some of them are conflicting. Here is my two cents:
Everything I have read (5 different sources) and my pediatrian says that the milk intake stays the same! The food is just to get them used to it, or as a lot of people said, a sensory experience. Until they are 10 months to one year old (depending on what you read), the formula is still the main nutrition. Nutrition is important not just for thier body growth, but BRAIN growth as well.
Since formula is still the main nutrition, I really didn't want my baby to fill up on food and not want formula. I always give her the bottle first, then about 30 minutes later the food. She still is a really good eater, so don't worry about your baby being too full for food.
Lastly, I would say that most babies, most nights, should be able to go through the night at 6 months without feeding. BUT if you just starting food, WAIT TO CHANGE the night feeding, you might find that it just goes away on it's own cause food is harder for them to digest so they stay full longer. If, after solids, that night feeding doesn't go away I recommend the book "sleepeasy solutions".
Good luck!