Once you're done, you're done. NHSC would like you to stay where you are at serving the community in your primary care specialty, as that is the whole goal of the program (not to address healthcare shortages not so much in 2-4 year stints of your repayment, but in the idea that a percentage of providers will stay.) They help make this ideal through the option to switch to loan repayment. But when your contract is over, you are free to move on.