Beer kit yeast is made to be almost armagheddon-proof, which means it can be stored at room temperature for at least its listed shelf life. Because there is generally amply sufficient yeast in the packet (much more than needed) you almost certainly have enough viable yeast left to ferment your beer, so you should be fine. I have brewed with expired yeast without problems. One word of caution, though: a lot depends on how the yeast has been stored. Higher temperatures reduce its shelf life significantly. So when in doubt err on the side of caution.
That said, personally I would replace kit yeast with a better yeast in any case because it will give you a much better beer. Kit yeast is cultivated to robust and idiot-proof (read: it will keep forever and do more or less the same job over a wide range of temperatures, oxygen levels and what not, but that comes at the price of a more neutral and far less interesting flavor profile. It will ferment your beer, but that's about it: it won't contribute significant quality with flavor components formed by the yeast during fermentation. A "proper" beer yeast (Fermentis, Lallemand, etc.) for the style you're brewing is always a good way to improve a kit beer.