"All Unix filesystems will ensure the move operation is atomic."

This is false, but most fs will. However, there is a lot of fs calls you have to make that you probably don't know about to make the fs operations atomic.

PS The way you propose is probably the hardest way to do an atomic FS operation. It will have the highest probably of failure and have the longest period of operations and service disruption. There is good reason we move rows one at a time or in batches sized to match OS buffers.