While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Under Armour Inc (NYSE:UAA) gained 2% this morning after Jefferies upgraded the athletic apparel specialist from hold to buy.

So what: Along with the upgrade, analyst Randal Konik boosted his price target to $65 (from $50), representing about 27% worth of upside to yesterday's close. So while contrarian traders might be turned off by Under Armour's price pullback in recent months, Konik's call could reflect a sense on Wall Street that its long-term growth prospects are becoming too cheap to pass up.

Now what: Jefferies believes that Under Armour's top line can exceed $15 billion over the next 10 years. "We conducted a survey of 2,000+ parents across the country to ascertain their children's activewear preferences," Konik said. "Our work confirms a strong and growing bias among youths for the UA brand, which ranked second only to Nike in terms of 'coolness' and preference. ... As a relatively small company (~$3 billion in annual revenue vs ~$28 billion for NKE) in an up-trending category, we see exceptional opportunities for UA to expand." When you couple that upbeat outlook with the stock's recent sluggishness -- still off about 20% from its 52-week highs -- it's tough to disagree with Jefferies' call.