Transplanting Seedlings

With the cold weather setting in, I'm doing a few things backwards. Usually one starts seedlings indoors and then transplants them outside. I'm doing the opposite. I dug up lettuce seedlings that are not making much progress outside and am moving them inside. When I went out to transplant them into a window box, I waited until 10 in the morning so that the chill would be out of the air. However, when I went around to the garden I found it still in the shade of the house and covered with frost. I attempted to dig up the lettuce, but the ground was pretty frozen for the first few layers.
I waited an hour and came back when the sun had melted the frost and warmed the soil a bit. As it was, I was pulling up chunks of frozen soil, which gave the advantage of holding the roots and soil together. I filled my window box mostly with soil from the garden, placed the seedlings on top, and then added some potting soil to the mix.
Though my south-east facing windows don't get full sun the warmth should give the lettuce an advantage for growth. It seems as though the garden isn't getting much sun these days anyways. In another experiment in indoor gardening, I've also planted lettuce seeds around my potted swiss chard. We'll see if they sprout.