Not a bad idea. I have, many times, used the "flip method" of building raised beds. I remove the sod and flip the pieces over then pile soil and compost on top.

The issue that I can see with your plan is this. When you place a ring of soil around the root system of a new plant, water becomes a factor. Especially on a slope. Plant roots will come to the surface to find water. If there is not deep saturation of the roots, then they will grow upward to find it. That happens easily in our clay soil because the water sheets right off the slope. We never want to encourage roots to surface when the plant is young and fragile. When it rains, they will no longer be bricks, but a mound of clay, soil and decomposing grass. Those young roots may actually head upward into your walls looking for water. They also will not serve as a weed barrier for long. The seeds of dandilions etc. that float around this time of year will LOVE the brand new open soil that is exposed.

Crabgrass is a pain in the grass. It spreads easily by seed. Because of it's existance in your sod, I would not compost those pieces.

The good news is, you have them flipped upside down and they have been baked in the sun. More than likely, you have killed the grasses and smothered the seeds. Leave your walls in place and in a few weeks take a flat edged shovel and chop them up then rake, mulch or plant grass seed on top.

Keep your new shrubs and trees well watered and be sure of full saturation with each watering. Less frequent deep waterings are much better than 10 minutes a day with a spray nozzle on your hose. Use your walls for that purpose. Place a garden hose inside of your walls and allow a trickle to fully saturate the root balls. If you can't get a hose there, use a bucket and slowly pour the water. Don't pour it so fast that it sheets off the surface. Take your time.

Know the water requirements of the plants you have in place. Water accordingly and your walls will be of help to you. But I would chop them up after their purpose is served and mulch or seed.

That is just what I would do. Others may have a better suggestion from their experience.

Good luck, and for future reference, covering the sod with black plastic for a few weeks prior to removal will kill many of the seeds and grasses that you don't want to spread to other areas when you relocate it.