I think it's better to move early if it's possible. The more involved a child is with friends, and in school the harder it will be to move. Kids are resilient and at an early age can adapt easily to change. The older and more involved with others they are when the move takes place, the more likely there will be a sense of trauma to overcome.
When my youngest daughter was 2 we moved to L.A. When we had an earthquake, we returned home to live after 5 months. She doesn't even remember the events. She was mostly involved at this time in her life with home, her sister and her parents. She hardly noticed that she had moved, except she was convinced that the witch from the Disneyland ride about Snow White was the culprit when the earth moved.