This series, brought to you by Yahoo! Finance, looks at which upgrades and downgrades make sense, and which ones investors should act on. In today's headlines, LabCorp (NYSE: LH) earns an upgrade, Lumber Liquidators (LL -1.32%) gets a downgrade, and prospects for Trina Solar (NYSE: TSL) dim, as one analyst cuts its price target. Let's dive in.

LabCorp -- it's alive!
First up, diagnostic tests specialist Laboratory Corporation of America received an upgrade to "buy" this morning from research firm WallachBeth. WB's buy thesis hinges on LabCorp's ability to garner new business as baby boomers' need for medical treatment increases, increasing demand among the elderly, and the Obamacare law going into effect, expanding usage of diagnostic tests more generally. WB assigns a $97 price target to the stock, which currently sells for closer to $85 -- suggesting 14% upside from today's prices.

But here's the thing: most analysts see the same trends WallachBeth is pointing to, and think they only add up to about 11.6% annualized profits growth for LabCorp over the next five years. Relative to the stock's 15.3 P/E ratio, that doesn't look like enough growth to justify the stock price. Plus, LabCorp is carrying about $2 billion in debt, net of cash on hand, meaning it's actually a bit more expensive than it looks on the surface.

Add it all up, and it's hard to see the stock as anything better than fairly priced at today's levels. Fact is, it might even be too expensive to own.

And speaking of expensive...
Have you looked at Lumber Liquidators lately? Buoyed by a revival in the housing market, the stock's more than tripled in price over the past year, and now carries a price-to-earnings ratio of more than 36. That's quite a pretty penny to pay, even with long-term growth estimated at 15% per annum.

Free cash flow at Lumber Liquidators is weak, too, lagging reported net income by more than 25%, and giving the stock an even richer price-to-free-cash-flow ratio than its already high P/E. So it's little wonder that analyst Stifel Nicolaus is starting to have doubts about the valuation. Recently, the company reported a blockbuster Q3, and raised its earnings outlook for the rest of the year. But while many investors think of this as "good news," and a reason to buy, Stifel is viewing the enthusiasm over Lumber Liquidators as a sign that it's time to make for the exits.

The analyst downgraded LL to "sell-sell" this morning. Given the valuation, I can't argue with that call at all.

Terrible Trina
Completing the day's trifecta of terrible stocks, Chinese solar operator Trina Solar turned GAAP negative for earnings last year, and hasn't looked back since. For four straight quarters, it's done nothing but lose money -- $92 million last quarter alone. And if you can imagine this, the situation's actually getting worse.

Yesterday, Trina warned investors that solar module shipments probably came in lower than expected in Q3, with as few as 375 megawatts worse of modules shipped, versus a previous high guidance of 480 megawatts. Topping off the bad news, Trina advised that "overall gross margin" for its products will be "between 0% and 1.5%."

Read that again. Gross margin will be at best 1.5%. That's before factoring in operating costs, tax -- anything but the raw cost of the materials going into Trina's modules. That's truly horrible news for Trina, yet the most analyst Ardour Capital is willing to do about it today is reduce its price target a couple bucks (to $4), and maintain a "hold" rating on the stock.

Suffice it to say, I disagree with both parts of that decision. With more than $500 million in net debt on its books, and a business centered on selling products for less than the cost of making them, Trina is no "hold." In any logical world, this stock deserves to be sold.

Fool contributor Rich Smith has no positions in the stocks mentioned above. The Motley Fool owns shares of Laboratory of America and Lumber Liquidators. Motley Fool newsletter services recommend Laboratory of America and Lumber Liquidators.