Are you in California? If you are then you can request the court by petition. If you're working on a visa you really ought to ask your lawyer as to whether its prudent to change your ne during the visa process.
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Last names are not automatically changed in California, you must start using your new name. Your marriage certificate should serve as adequate proof for institutions needing proof.
But Ms. Johns is wise to point out you should not change anything until you've spoken with your attorney.
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