H1B gets extended while current H1B is still valid

This might be a very stupic question, but to be one the safe side, I'm still asking:

My current H1B will expire on Sep 15 2014, my employer just got it renewed with premium process, so the new H1B says valid from Sep 16 2014 to Sep 15 2017. I plan to travel to Canada and come back before Sep 15 2014, (I'm Canadian citizen). Which H1B package should I use ? The one expired on Sep 15 2014, right ? Many thanks