For short trips of 30 days or less, to Canada, Mexico or adjacent islands excluding Cuba, it seems you can re-enter on your expired visa and your new I-94, once your extension of stay has been approved. 22CFR41.112(d)[1]. If it were me I would get a new visa though.

http://www.gpo.gov/fdsys/pkg/CFR-2010-title22-vol1/xml/CFR-2...