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 United Continental Holdings (NASDAQ:UAL) climbed 3% this morning after Goldman Sachs upgraded the airline operator from neutral to buy.

So what: Along with the upgrade, analyst Tom Kim boosted his price target to $44 (from $35), representing about 20% worth of upside to yesterday's close. While contrarians might be turned off by United Continental's steady share-price rise over the past month, Kim believes there's plenty of room to fly given the company's strong earnings potential and relatively cheap valuation.

Now what: Goldman expects United's 2014 earnings per share to almost double in 2014 to $4.87. "The carrier has been an under-earner that we think is poised to post outsized earnings growth in 2014," noted Goldman. "The stock is undervalued versus its global network peers. ... As investor confidence in UAL's earnings power increases, we expect the discount at which the stock trades versus its peers narrows." But while United shares might keep flying in 2014, the airline industry's extremely intense nature, coupled with the stock's already high altitude (now double its 52-week lows), make it a far less attractive long-term holding.