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 AutoZone (NYSE:AZO) gained about 2% today after Stifel upgraded the auto replacement parts retailer from hold to buy.

So what: Along with the upgrade, analyst David Schick planted a price target of $570 on the stock, representing about 8% worth of upside to yesterday's close. So while contrarian traders might be turned off by AutoZone's price strength over the past year, Schick's call could reflect a sense on Wall Street that its recent operating momentum gives the stock some solid room to run.

Now what: After meeting with AutoZone's management, Stifel is rather bullish about AutoZone's near-term risk/reward trade-off. "AZO experienced stronger trends in the Northeast and Midwest last Q driven by colder winter weather and easier compares y/y," said Schick. "The West Coast underperformed relative to strength in these regions (not as much upside from weather). Management remains optimistic about a continuation of parts breakage with hot summer weather." When you couple AutoZone's still-hefty debt load with its near-20 P/E, however, I'd hold out for a wider margin of safety before buying into that bullishness.