Yes you can substitute 1 cup yogurt for 1 cup sour cream. May not work for all recipes but should work if you use a plain lowfat(not no fat). you still want to try to get the same fat content as the sour cream and the thickness.
Yes! In many cases yogurt can be substituted for sour cream when cooking to make the recipe healthier. For example, you can make a healthy onion dip with plain Greek yogurt instead of sour cream. It tastes the exact same and is much healthier. I would not use a sweetened yogurt.
It depends upon how much sour cream is being used and why the recipe calls for it. Usually, this substitution will work, but it sometimes will not, especially if the recipe calls for a large amount of sour cream.