http://www.travel.state.gov/revals.html
If your existing visa is still valid, it will not be considered for re-issuance until the remaining validity is within sixty days of expiring. Please note that you do not require a new visa merely because you have changed employers and received a new I-797 petition from the Immigration and Naturalization Service (INS). If your current visa is still valid, you may reenter the US with that visa as long as you are in possession of a valid I-797, even though the visa contains the information from your former employer.
If your existing visa is still valid, it will not be considered for re-issuance until the remaining validity is within sixty days of expiring. Please note that you do not require a new visa merely because you have changed employers and received a new I-797 petition from the Immigration and Naturalization Service (INS). If your current visa is still valid, you may reenter the US with that visa as long as you are in possession of a valid I-797, even though the visa contains the information from your former employer.