FSA - if u do not use it, u lose it. However, if u underpay, u do not need to pay back.
401k - u can either leave in your old company if they have good plan and allow u to do so, or roll over to your new employer's 401k, or roll over to your personal rollover IRA like vanguard and etc.
401k - u can either leave in your old company if they have good plan and allow u to do so, or roll over to your new employer's 401k, or roll over to your personal rollover IRA like vanguard and etc.