In the year of your tax residency status change, you are generally considered as a Dual-Status taxpayer. The U.S. income tax return you must file as a dual-status alien depends on whether you are a resident alien or a nonresident alien at the end of the tax year.
If you are a dual-status taxpayer who becomes a resident during the year and who is a U.S. resident on the last day of the tax year, you must file Form 1040. Write "Dual-Status Return" across the top of the return. Attach a statement to your return to show the income for the part of the year you are a nonresident. You can use Form 1040NR or Form 1040NR-EZ as the statement.
If you are a resident alien at the beginning of the year and a nonresident at the end of the year, you can file Form 1040NR or Form 1040NR-EZ and attach Form 1040 as the statement. Be sure to mark "Dual-Status Statement" across the top.
For additional information, refer to the IRS web page Taxation of Dual-Status Aliens.
Please note, dual status does not refer to your citizenship, only to your U.S. tax residency status.