Your milk is your A choice, which is a recommended amount to use each day. Natural yogurt is a free food, which is classed as an unlimited food, so you can eat as much as you want each day.
If you don't use all your milk, it isn't an issue. You don't have to use all of it, it's a recommended limit.
And if you want to have yogurt, you can have that as well as having your milk, you don't need to choose between them
Does that make sense?