This.

This can be a big deal. I changed from L1B to L1A to H1B (without leaving the country) then when I wanted to leave the country the legal advice that I received (IANAL - TIJAA[1]) was that to be safe I should return to my home country to get the new visa stamp, the Vancouver embassy might legitimately decide not to apply it - at which point I'd be stuck in Canada needing a plane ticket to the UK in a hurry and have the possibility of a multi-week wait for an appointment at the London embassy...

I waited on my green-card application to get to the point at which I could get an Advanced Parole[2] document and traveled on that instead, but this did mean: a) several years in which I couldn't travel outside the US; b) repeated lectures from border officials that the AP document was for "serious travel" only and that snowboarding trips to Whistler didn't count.

YMMV

[1] This Is Just An Anecdote [2] http://en.wikipedia.org/wiki/Advance_parole

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...