Assuming that the person is not going to be in the elite of the command economy, their quality of life would drop. They would have less access to material goods and they would have less freedom.
In a command economy, fewer consumer goods are available. The government determines what is made and that tends to mean that there is less innovation in consumer goods. It also tends to mean that there are fewer incentives to make good quality goods. This would mean a decline in material quality of life.
In a command economy, there is generally less economic freedom as well. People are not free to do as they wish with their property and their money. This would be a major change from life in the US.