A person temporarily present in the United States may change status from one temporary (nonimmigrant) status to another temporary status. For example, a foreign student who acquires a job in the United States may, upon graduation, change status from a student visa to a temporary work visa.
A change of status is a different idea than “adjustment of status.” An adjustment of status describes the process used by a foreign national who is physically present in the United States to become a lawful permanent resident. In contrast, change of status deals with getting another type of temporary – not permanent – visa.
If you are eligible for change of nonimmigrant status, you may change the status at a US consulate abroad or in the United States.
The key is that you must properly file an application to change status before the authorized stay expires. If you timely file your change of status, then you are permitted to remain in the United States while your application is pending, even the application is decided only after your stay. If you fail to file on time, then you have some wiggle room for an exception, but it is not easy.
If you are admitted to the United States even though you were unlawfully present during a previous stay, then you are not eligible for change of status.
There are some tricky exceptions for J visa holders, K nonimmigrant fiancés, and if you are admitted as a nonimmigrant without a visa under the visa waiver program.
If there is any complications involved with your status, see an immigration lawyer.