Yes, you can replace an existing light fixture with a ceiling fan. In order to do this, you must ensure that the electrical box is securely mounted to a ceiling joist or support bracket. You also need to use a ceiling fan mount, which is almost always included when you purchase a ceiling fan. If necessary, you can buy a ceiling fan mount separately. Before inspecting the electrical box attachment, be sure to turn off the electricity to the room at the circuit breaker. If the electrical box is not securely fastened, you will need to install a support brace. This is because ceiling fans are much heavier than most lighting fixtures and you run the risk of having a ceiling fan fall on you or someone else without the proper support.