If you are in the U.S., you must maintain lawful status at all times. If you are in the U.S. in a temporary, nonimmigrant status, you must therefore extend or change your status, or leave the country, by the expiration date of your classification. There are few exceptions to this rule.
The amount of time you are given under an extension will vary depending upon the terms of your status. Additionally, it is important to note that if you have violated the terms of your status, such as by working without authorization, you will not likely be able to extend or change your status.
If you are lawfully in the U.S. under one classification of nonimmigrant status and wish to change to a different nonimmigrant classification, you may be eligible to apply to change your status. To be eligible you must not be out of status (i.e. you must have an unexpired, valid status).
Another requirement is that you must not be within one of the classes of nonimmigrants that are ineligible for change of status. Some of the nonimmigrant visa categories that are not permitted to change status include, Visa Waiver Program, C visas (transit), D visas (crewman), K visas (Fiancés) and S visas (informants). Foreign nationals in J-1 status are not eligible for change of status unless they are not subject to the two year home residency requirement or have received a waiver of same.
Finally, to change status you must be admissible to the U.S., and you must not have had the preconceived intent to seek the change of status before your original admission.