What do you mean by "switch"? Any tile that falls within the boundaries of two or more cities is workable by any of those cities. You can't assign a worker from one city to a tile that's currently being worked by another. So if you're in the city screen of City A, you cannot assign workers to tiles being worked by City B. What you need to do is go to the city screen for City B, reassign the worker from the tile in question, and then go back to City A and work the tile.