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 Petroleo Brasileiro Petrobras SA (NYSE:PBR) rallied about 2% today after Credit Suisse upgraded the Brazilian oil giant from underperform to neutral.

So what: Along with the upgrade, analyst Vinicius Canheu reiterated his price target of $14, representing about 30% worth of upside to yesterday's close. So while momentum traders might be turned off by the stock's weakness over the past several months, Canheu's call suggests growing sentiment on Wall Street that Petrobras is becoming too cheap to pass up.

Now what: According to Credit Suisse, Petrobras' long-term risk/reward trade-off is much improved. "Even in a scenario where the FX depreciates further to 2.6x and we see 'only' 10% price hikes in 2015, PBR could be a close to 6.0x PE stock on reasonably conservative earnings," said Canheu. "With the company recently going to the debt markets and raising c.$13bn YTD, capital needs are nearly solved for 2014, decreasing the fears of further equity dilution this year (though cheap debt also has an indirect psychological negative effect of removing the sense of urgency to increase gasoline and diesel prices)." When you couple Petrobras' firmed-up financial position with its still-ice cold stock price -- off about 45% from its 52-week highs -- it's tough to disagree with Credit Suisse's upgrade.