The name Walmart (WMT 0.62%) doesn't usually conjure up images of upscale shopping, but the discount supermarket giant, which happens to be the largest company in the U.S. by sales, has recently switched tracks to appeal to precisely this demographic.I don't think Walmart is going to pivot away from its discount roots, but it's successfully embracing a broader swath of shoppers, and that could lead to significant growth in the next few years. Here's what's going on and why Walmart is winning.What's going on ...