What was the long time change because of the frozen food invention?
I'll answer this question as though it read, "What was a result of the invention of frozen food"?
Frozen food, like most inventions, sought to provide convenience. The benefit of freezing food is that it "keeps" for much longer than leaving it out or even in the refrigerator.
Frozen vegetables are often chopped before they're frozen, making it easy to prepare what is needed and keep the rest frozen.
Side dishes (scalloped potatoes, for example) can be prepared and then frozen. Heating a frozen side dish greatly reduces the amount of time needed to prepare an entire meal.
Even entire meals are available frozen. While often containing high amounts of sodium and other preservatives to maintain freshness, frozen food had greatly reduced the amount of time people spend preparing meals. Many will argue that, in exchange for the convenience, we have given up a good deal of the nutritional value of the food.
One of the problems that people have always had was the preservation of food. Imagine what the ancients had to do to keep their food, or what people had to do even before the invention of refrigeration. Salt was answer.
In the light of this, the ability to freeze foods not only helped in the area of convenience, but also provided the ability to preserve food for many different purposes, even for months.
You might also want to think about this from the side of farming and marketing. Now farmers do not have to waste a great harvest. They can reap and freeze. Also, those who market food are now able to sell food to people in different season, because they are able to freeze things.