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 Micron Technology (NASDAQ:MU) rallied 2.5% this morning after Bank of America upgraded the semiconductor company from underperform to buy.

So what: Along with the two-notch upgrade, analyst Simon Dong-je Woo boosted his price target to $40 (from $22), representing about 35% worth of upside to yesterday's close. So while contrarian traders might be turned off by Micron's steady price strength over the past year, Dong-je Woo's call could reflect a sense on Wall Street that its growth prospects still aren't fully baked into the valuation.

Now what: According to B of A, Micron's risk/reward trade-off is particularly attractive at this point. "In our view, key catalysts are solid chip pricing environment (tight supply) and cost competitiveness," said Dong-je Woo. "Lower cyclical risk and higher shareholder return [should] lead to bigger EVA (WACC down vs ROE up)." Of course, when you couple Micron's still-significant debt load with its red-hot stock price -- now up a whopping 150% from its 52-week lows -- I'd hold out for a much wider margin of safety before betting on that bullishness.