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

What: Shares of Amazon.com, (NASDAQ:AMZN) gained 2% this morning after Goldman Sachs upgraded the online retail gorilla from buy to conviction buy.

So what: Along with the upgrade, analyst Heath Terry planted a price target of $400 on the stock, representing about 20% worth of upside to yesterday's close. So while momentum traders might be turned off by Amazon's year-to-date price weakness, Terry's call could reflect a sense on Wall Street that the worries about its growth trajectory are becoming overblown.

Now what: According to Goldman, Amazon's risk/reward trade-off is one of the best in the sector. "While concerns over Amazon Web Services (AWS) pricing, slowing Media growth, and ongoing margin pressures are likely to persist, we believe -- given its track record of delivering consistently high returns on invested capital -- the ongoing investment in fulfillment and infrastructure will widen Amazon's competitive lead through initiatives such as Fresh, AWS, digital media, and faster fulfillment and product availability, driving incremental growth and, ultimately, profitability," said Terry. When you couple that upbeat outlook with Amazon's rock-solid balance sheet and still-sluggish stock price, it's tough to disagree with Goldman's bullishness.