BOSTON ( TheStreet) -- Here are three critical rating changes from TheStreet's stock model.3. The model upgraded precious-metals purchaser Silver Wheaton ( SLW) to "buy." The numbers: Silver Wheaton swung to a fourth-quarter profit of $51 million, or 15 cents a share, from a loss of $54 million, or 22 cents, a year earlier. Revenue tripled to $91 million. The operating margin widened from 33% to 56%. Silver Wheaton holds $228 million of cash and $503 million of debt. The stock: Silver Wheaton has more than doubled during the past year, outpacing major benchmarks. The stock trades at a price-to-book ratio of 3.1, a 25% discount to the industry average. It is expensive based on sales and cash flow. A beta of 2 suggests the shares swing more than the market. 2. The model downgraded utility Dominion Resources ( D) to "hold." The numbers: Dominion swung to a fourth-quarter loss of $4 million, or 1 cent, from a profit of $353 million, or 60 cents, a year earlier. Revenue decreased 22% to $3.3 billion. The operating margin fell into shallow negative territory. The company holds $48 million of cash and $18 billion of debt. The stock: Dominion Resources has gained 39% during the past year, lagging behind U.S. indices. The stock sells for a price-to-projected-earnings ratio of 12, a 24% discount to the peer-group average. It is expensive based on book value, sales and cash flow. The shares offer a 4.7% dividend yield.