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 analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Plug Power (PLUG -1.61%) climbed 4% today after Cowen upgraded the fuel-cell provider from perform to outperform.

So what: Along with the upgrade, analyst Robert Stone lowered his price target to $6 (from $7.50) on higher expenses and new shares, representing about 60% worth of upside to yesterday's close. So while momentum traders might be turned off by Plug Power's sharp pullback in recent weeks, Stone's call could reflect a sense on Wall Street that the company's growth prospects are becoming too cheap to pass up.

Now what: According to Cowen, Plug Power's risk/reward trade-off is rather attractive at this point. "YTD bookings of $80MM (2x vs. 2013) point to a steep ramp from Q2 and profitability in Q4," said Stone. "Our factory visit yesterday (5/14/14) confirmed a high level of activity. A strong cash position should support expansion into hydrogen generation, Asia, and new product segments." When you couple today's rally with the infrastructure risk still surrounding Plug Power's shipments, however, I'd hold out for a wider margin of safety before buying into those prospects.