you need to go for stamping again.... on the I797 basics they can't extend you stay .... hope this helps you .
two terms Stamped visa and I797 I94 bothe are different on I797 it is clearly mention check some where "this is not a visa" stating ..... when ever we leave the country we need to have a valid Visa .. then only we can get in ..... I797, OPT EADs, H4EADs not a visa .......