We consider the customers' equilibrium strategy and socially optimal strategy in a single server Markovian queueing system with changeable service rates controlled by a threshold. When a customer arrives at an empty system, he is served by the server at a lower service rate. When the queue length reaches the threshold, customers are served at a high service rate. The optimal joining strategies of customers are studied under two information scenarios. The first scenario, where the server' state and the queue length are observable, is called a fully observable case. The second scenario, where the system state is not observable, is called an unobservable case. We analyze the steady-state distribution and performance measures of the system, and derive the equilibrium strategy. Finally, we compare the equilibrium strategy with socially optimal strategy via numerical examples.