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 The Boeing Company (NYSE:BA) slipped nearly 3% today after Oppenheimer downgraded the aerospace and defense giant from outperform to perform.

So what: Along with the downgrade, analyst Yair Reiner removed his $140 price target, suggesting that he sees limited upside, and possibly even significant downside, from Boeing's current levels. While momentum traders might be attracted to the stock's steady climb in 2013, Reiner believes that Mr. Market might be underestimating some headwinds working against the company.

Now what: Oppenheimer sees Boeing's risk and reward trade-off as pretty balanced at this point. "While we still see Boeing's FCF improving over the next two years, we believe investors are overlooking how a convergence of factors are aligning to make 2015 a medium-term peak," noted Oppenheimer. "Yes, 787 investment should (should) top out in 2015, but the call on cash is likely to be quickly replaced by investments in the 737MAX and 777x." With Boeing shares still up about 90% from their 52-week lows, and trading at a 20-plus P/E, waiting for a wider margin of safety certainly seems prudent.