Eligibility Requirements for a Tourist Visa: Foreigners wishing to visit the United States for leisure or tourism are usually eligible to receive a B1/B-2 visa. These are "visitor visas," and they are mandatory for citizens of countries not included in the Visa Waiver Program. However, even citizens of countries included in the visa waiver program need to have a valid B-12 visa if they plan to stay in the United States for more than 90 days, if they need to modify the status of their nonimmigrant visa, or change status to “Permanent resident” (Green Card), after entering the country.
Medical treatment with the B1/B2 visa: In addition to tourism and business, the government also issues B1/B-2 visas to people coming to the United States for medical treatment. The application process is similar to that of a tourist visa, but there are additional documents that must be submitted to establish that the applicant qualifies for a permit.
The term of the visit with the B1/B2 visa: People admitted to the United States with a B1/B-2 tourist visa usually have permission for a 6-month stay. That is the maximum length of a visit for visitors with a tourist visa. The immigration officer determines how long each visitor is allowed to stay in the country. Most visitors have their passports stamped with a 6-month stay authorization; however the immigration officer is entitled to issue a shorter visit based on each case. Upon entry into the United States, the foreign visitor has the right to request an extension of the stay.
Modification of Visa Status: Individuals who enter the United States with a B1/B2 visa are generally eligible to change their status to permanent resident (Green Card holder) if they qualify (H-1B, H-2B, E-1, E-2, E-3, etc.), student (under F-1 Student Visa), or another nonimmigrant status, such as temporary worker. Individuals who enter the United States under the Visa Waiver Program are not eligible to change their situation. The option to change status is the main advantage of having a nonimmigrant visa, such as the B1/B2 visa if you are part of the Visa Waiver Program.
About US Visa Renewals: B1/B2 Visas cannot be renewed or extended in every embassy or consulate. Some of those embassies or consulates don´t have a formal application for "visa renewal." If your visa has expired, you must apply a new one. Many applicants think they can easily renew or extend an American Visitor Visa because they already had a previous visa. However, this is not the case always, since many US embassies and consulates consider renewals or extensions of a visa the same as a new visa application. Usually, the same requirements and application procedures apply. However, some US embassies and consulates have "Procedures for re-use and reissue of visas" which makes it easier and faster to request a new visitor visa.
Eligibility and Requirements for Visa Renewal: Anyone who previously had a valid visa or has a permit about to expire has the right to apply for a new visitor visa for the United States. There is no specific process to renew the visa en some embassies and consulates, so you must submit a new visa application; however, some US embassies and consulates have special procedures that are available to applicants who previously had a valid visa or have a permit about to expire. Some people call this process "Visa Reuse and Reissue Procedures." One of the main advantages of visa renewals is that the applicant may not be required to attend the interview for the visa, especially if the person has had a US visa for several years, has entered and exited the States United States and never exceeded his stay in the country.
Can I travel with a valid visa on an expired passport? Unless previously canceled, a visa is valid until its expiration date. Therefore, if you have a US Visitor Visa on an expired passport, you can use it along with a new, valid passport to travel to and enter the United States.