The problem with cooking soups with milk, cream, or cheese in them for a long time or at high heat is that the proteins can curdle, so the milk will become grainy, or the cheese will become stringy and won't melt and mix into the soup. You could get it all made to the point of adding the cheese, and then just wait and do it before dinner. If this is the Velveeta recipe, you might be okay, because I think that one will stay creamy, but I'm not positive. I haven't ever cooked with Velveeta.
Life's a journey, not a destination.
It's easier to stay on plan than to get back on plan.