Q: would replacing juice that has sucrose and fructose with pure juice that only has naturally occuring sugars help me lose weight?

A: It's a good step to make in general, but it will make only a very small difference in weight loss. If you're looking to lose weight (or fat, more specifically) you will want to avoid fruit juices altogether, whether 100% natural or processed. Instead you should drink lots of plain water and eat the actual fruits and even those in moderation when trying to lose fat (no more than 2-3 pieces of fruit a day, preferably in the morning and early afternoon).

A good weight loss diet will emphasize vegetables as the primary source of carbs along with legumes (beans and lentils), a few pieces of fruit and/or berries, and some whole grains (like old-fashioned oatmeal for breakfast). And the vegetables here should NOT include corn or potatoes, but instead be less starchy ones like broccoli, spinach, lettuce, peppers, onions, tomatoes, cucumbers, cauliflower, etc. For proteins you'd eat the usual fish, meats, poultry, eggs, cottage cheese (but limit other dairy). And you'd get fats from some of the same protein sources plus nuts and olive oil.

Things like pasta, white bread and white rice (and bread/rice in general -- eat whole grains and legumes instead), pizza, commercial salad dressings, heavily processed foods, sweets, sodas, juices, fast/junk food, and alcohol should all be kept to an absolute minimum for optimal fat loss.