If you have cash, there are no taxes involved in the act of transferring it from a US bank to an overseas bank.
If you retain your US citizenship, you are required to report and pay US income tax on all your world-wide income, no matter where you live. This means that any investments you make with the money overseas, or any interest paid to you by banks, must be reported on a US tax return. Even though overseas banks and investment firms might not issue 1099-INT or 1099-B statements, you still are supposed to report the income and pay the tax. If you are also taxed on the same credit in a foreign country, the US will give you a credit or deduction against that tax, but you still have to file.
Just the act of transferring your own money from one place to another is not taxable.
